Add doc about FST-based CTC forced alignment. (#1482)

docs/source/conf.py

@@ -98,4 +98,6 @@ rst_epilog = """
 .. _Next-gen Kaldi: https://github.com/k2-fsa
 .. _Kaldi: https://github.com/kaldi-asr/kaldi
 .. _lilcom: https://github.com/danpovey/lilcom
+.. _CTC: https://www.cs.toronto.edu/~graves/icml_2006.pdf
+.. _kaldi-decoder: https://github.com/k2-fsa/kaldi-decoder
 """

docs/source/docker/intro.rst

@@ -34,6 +34,8 @@ which will give you something like below:
 
 .. code-block:: bash
 
+  "torch2.3.1-cuda12.1"
+  "torch2.3.1-cuda11.8"
   "torch2.2.2-cuda12.1"
   "torch2.2.2-cuda11.8"
   "torch2.2.1-cuda12.1"

docs/source/fst-based-forced-alignment/diff.rst

@@ -0,0 +1,41 @@
+Two approaches
+==============
+
+Two approaches for FST-based forced alignment will be described:
+
+  - `Kaldi`_-based
+  - `k2`_-based
+
+Note that the `Kaldi`_-based approach does not depend on `Kaldi`_ at all.
+That is, you don't need to install `Kaldi`_ in order to use it. Instead,
+we use `kaldi-decoder`_, which has ported the C++ decoding code from `Kaldi`_
+without depending on it.
+
+Differences between the two approaches
+--------------------------------------
+
+The following table compares the differences between the two approaches.
+
+.. list-table::
+
+ * - Features
+   - `Kaldi`_-based
+   - `k2`_-based
+ * - Support CUDA
+   - No
+   - Yes
+ * - Support CPU
+   - Yes
+   - Yes
+ * - Support batch processing
+   - No
+   - Yes on CUDA; No on CPU
+ * - Support streaming models
+   - Yes
+   - No
+ * - Support C++ APIs
+   - Yes
+   - Yes
+ * - Support Python APIs
+   - Yes
+   - Yes

docs/source/fst-based-forced-alignment/index.rst

@@ -0,0 +1,18 @@
+FST-based forced alignment
+==========================
+
+This section describes how to perform **FST-based** ``forced alignment`` with models
+trained by `CTC`_ loss.
+
+We use `CTC FORCED ALIGNMENT API TUTORIAL <https://pytorch.org/audio/main/tutorials/ctc_forced_alignment_api_tutorial.html>`_
+from `torchaudio`_ as a reference in this section.
+
+Different from `torchaudio`_, we use an ``FST``-based approach.
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+
+   diff
+   kaldi-based
+   k2-based

docs/source/fst-based-forced-alignment/k2-based.rst

@@ -0,0 +1,4 @@
+k2-based forced alignment
+=========================
+
+TODO(fangjun)

docs/source/fst-based-forced-alignment/kaldi-based.rst

@@ -0,0 +1,712 @@
+Kaldi-based forced alignment
+============================
+
+This section describes in detail how to use `kaldi-decoder`_
+for **FST-based** ``forced alignment`` with models trained by `CTC`_ loss.
+
+.. hint::
+
+  We have a colab notebook walking you through this section step by step.
+
+  |kaldi-based forced alignment colab notebook|
+
+  .. |kaldi-based forced alignment colab notebook| image:: https://colab.research.google.com/assets/colab-badge.svg
+     :target: https://github.com/k2-fsa/colab/blob/master/icefall/ctc_forced_alignment_fst_based_kaldi.ipynb
+
+Prepare the environment
+-----------------------
+
+Before you continue, make sure you have setup `icefall`_ by following :ref:`install icefall`.
+
+.. hint::
+
+   You don't need to install `Kaldi`_. We will ``NOT`` use `Kaldi`_ below.
+
+Get the test data
+-----------------
+
+We use the test wave
+from `CTC FORCED ALIGNMENT API TUTORIAL <https://pytorch.org/audio/main/tutorials/ctc_forced_alignment_api_tutorial.html>`_
+
+.. code-block:: python3
+
+  import torchaudio
+
+  # Download test wave
+  speech_file = torchaudio.utils.download_asset("tutorial-assets/Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav")
+  print(speech_file)
+  waveform, sr = torchaudio.load(speech_file)
+  transcript = "i had that curiosity beside me at this moment".split()
+  print(waveform.shape, sr)
+
+  assert waveform.ndim == 2
+  assert waveform.shape[0] == 1
+  assert sr == 16000
+
+The test wave is downloaded to::
+
+  $HOME/.cache/torch/hub/torchaudio/tutorial-assets/Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav
+
+.. raw:: html
+
+  <table>
+    <tr>
+      <th>Wave filename</th>
+      <th>Content</th>
+      <th>Text</th>
+    </tr>
+    <tr>
+      <td>Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav</td>
+      <td>
+       <audio title="Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+      <td>
+        i had that curiosity beside me at this moment
+      </td>
+    </tr>
+  </table>
+
+We use the test model
+from `CTC FORCED ALIGNMENT API TUTORIAL <https://pytorch.org/audio/main/tutorials/ctc_forced_alignment_api_tutorial.html>`_
+
+.. code-block:: python3
+
+  import torch
+
+  bundle = torchaudio.pipelines.MMS_FA
+
+  device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
+  model = bundle.get_model(with_star=False).to(device)
+
+The model is downloaded to::
+
+  $HOME/.cache/torch/hub/checkpoints/model.pt
+
+Compute log_probs
+-----------------
+
+.. code-block:: bash
+
+  with torch.inference_mode():
+      emission, _ = model(waveform.to(device))
+      print(emission.shape)
+
+It should print::
+
+  torch.Size([1, 169, 28])
+
+Create token2id and id2token
+----------------------------
+
+.. code-block:: python3
+
+    token2id = bundle.get_dict(star=None)
+    id2token = {i:t for t, i in token2id.items()}
+    token2id["<eps>"] = 0
+    del token2id["-"]
+
+Create word2id and id2word
+--------------------------
+
+.. code-block:: python3
+
+  words = list(set(transcript))
+  word2id = dict()
+  word2id['eps'] = 0
+  for i, w in enumerate(words):
+    word2id[w] = i + 1
+
+  id2word = {i:w for w, i in word2id.items()}
+
+Note that we only use words from the transcript of the test wave.
+
+Generate lexicon-related files
+------------------------------
+
+We use the code below to generate the following 4 files:
+
+  - ``lexicon.txt``
+  - ``tokens.txt``
+  - ``words.txt``
+  - ``lexicon_disambig.txt``
+
+.. caution::
+
+   ``words.txt`` contains only words from the transcript of the test wave.
+
+.. code-block:: python3
+
+  from prepare_lang import add_disambig_symbols
+
+  lexicon = [(w, list(w)) for w in word2id if w != "eps"]
+  lexicon_disambig, max_disambig_id = add_disambig_symbols(lexicon)
+
+  with open('lexicon.txt', 'w', encoding='utf-8') as f:
+    for w, tokens in lexicon:
+      f.write(f"{w} {' '.join(tokens)}\n")
+
+  with open('lexicon_disambig.txt', 'w', encoding='utf-8') as f:
+    for w, tokens in lexicon_disambig:
+      f.write(f"{w} {' '.join(tokens)}\n")
+
+  with open('tokens.txt', 'w', encoding='utf-8') as f:
+    for t, i in token2id.items():
+      if t == '-':
+        t = "<eps>"
+      f.write(f"{t} {i}\n")
+
+    for k in range(max_disambig_id + 2):
+      f.write(f"#{k} {len(token2id) + k}\n")
+
+  with open('words.txt', 'w', encoding='utf-8') as f:
+    for w, i in word2id.items():
+      f.write(f"{w} {i}\n")
+    f.write(f'#0 {len(word2id)}\n')
+
+
+To give you an idea about what the generated files look like::
+
+  head -n 50 lexicon.txt lexicon_disambig.txt tokens.txt words.txt
+
+prints::
+
+  ==> lexicon.txt <==
+  moment m o m e n t
+  beside b e s i d e
+  i i
+  this t h i s
+  curiosity c u r i o s i t y
+  had h a d
+  that t h a t
+  at a t
+  me m e
+
+  ==> lexicon_disambig.txt <==
+  moment m o m e n t
+  beside b e s i d e
+  i i
+  this t h i s
+  curiosity c u r i o s i t y
+  had h a d
+  that t h a t
+  at a t
+  me m e
+
+  ==> tokens.txt <==
+  a 1
+  i 2
+  e 3
+  n 4
+  o 5
+  u 6
+  t 7
+  s 8
+  r 9
+  m 10
+  k 11
+  l 12
+  d 13
+  g 14
+  h 15
+  y 16
+  b 17
+  p 18
+  w 19
+  c 20
+  v 21
+  j 22
+  z 23
+  f 24
+  ' 25
+  q 26
+  x 27
+  <eps> 0
+  #0 28
+  #1 29
+
+  ==> words.txt <==
+  eps 0
+  moment 1
+  beside 2
+  i 3
+  this 4
+  curiosity 5
+  had 6
+  that 7
+  at 8
+  me 9
+  #0 10
+
+.. note::
+
+   This test model uses characters as modeling unit. If you use other types of
+   modeling unit, the same code can be used without any change.
+
+Convert transcript to an FST graph
+----------------------------------
+
+.. code-block:: bash
+
+   egs/librispeech/ASR/local/prepare_lang_fst.py --lang-dir ./
+
+The above command should generate two files ``H.fst`` and ``HL.fst``. We will
+use ``HL.fst`` below::
+
+  -rw-r--r-- 1 root root  13K Jun 12 08:28 H.fst
+  -rw-r--r-- 1 root root 3.7K Jun 12 08:28 HL.fst
+
+Force aligner
+-------------
+
+Now, everything is ready. We can use the following code to get forced alignments.
+
+.. code-block:: python3
+
+  from kaldi_decoder import DecodableCtc, FasterDecoder, FasterDecoderOptions
+  import kaldifst
+
+  def force_align():
+      HL = kaldifst.StdVectorFst.read("./HL.fst")
+      decodable = DecodableCtc(emission[0].contiguous().cpu().numpy())
+      decoder_opts = FasterDecoderOptions(max_active=3000)
+      decoder = FasterDecoder(HL, decoder_opts)
+      decoder.decode(decodable)
+      if not decoder.reached_final():
+          print(f"failed to decode xxx")
+          return None
+      ok, best_path = decoder.get_best_path()
+
+      (
+          ok,
+          isymbols_out,
+          osymbols_out,
+          total_weight,
+      ) = kaldifst.get_linear_symbol_sequence(best_path)
+      if not ok:
+          print(f"failed to get linear symbol sequence for xxx")
+          return None
+
+      # We need to use i-1 here since we have incremented tokens during
+      # HL construction
+      alignment = [i-1 for i in isymbols_out]
+      return alignment
+
+  alignment = force_align()
+
+  for i, a in enumerate(alignment):
+    print(i, id2token[a])
+
+The output should be identical to
+`<https://pytorch.org/audio/main/tutorials/ctc_forced_alignment_api_tutorial.html#frame-level-alignments>`_.
+
+For ease of reference, we list the output below::
+
+  0 -
+  1 -
+  2 -
+  3 -
+  4 -
+  5 -
+  6 -
+  7 -
+  8 -
+  9 -
+  10 -
+  11 -
+  12 -
+  13 -
+  14 -
+  15 -
+  16 -
+  17 -
+  18 -
+  19 -
+  20 -
+  21 -
+  22 -
+  23 -
+  24 -
+  25 -
+  26 -
+  27 -
+  28 -
+  29 -
+  30 -
+  31 -
+  32 i
+  33 -
+  34 -
+  35 h
+  36 h
+  37 a
+  38 -
+  39 -
+  40 -
+  41 d
+  42 -
+  43 -
+  44 t
+  45 h
+  46 -
+  47 a
+  48 -
+  49 -
+  50 t
+  51 -
+  52 -
+  53 -
+  54 c
+  55 -
+  56 -
+  57 -
+  58 u
+  59 u
+  60 -
+  61 -
+  62 -
+  63 r
+  64 -
+  65 i
+  66 -
+  67 -
+  68 -
+  69 -
+  70 -
+  71 -
+  72 o
+  73 -
+  74 -
+  75 -
+  76 -
+  77 -
+  78 -
+  79 s
+  80 -
+  81 -
+  82 -
+  83 i
+  84 -
+  85 t
+  86 -
+  87 -
+  88 y
+  89 -
+  90 -
+  91 -
+  92 -
+  93 b
+  94 -
+  95 e
+  96 -
+  97 -
+  98 -
+  99 -
+  100 -
+  101 s
+  102 -
+  103 -
+  104 -
+  105 -
+  106 -
+  107 -
+  108 -
+  109 -
+  110 i
+  111 -
+  112 -
+  113 d
+  114 e
+  115 -
+  116 m
+  117 -
+  118 -
+  119 e
+  120 -
+  121 -
+  122 -
+  123 -
+  124 a
+  125 -
+  126 -
+  127 t
+  128 -
+  129 t
+  130 h
+  131 -
+  132 i
+  133 -
+  134 -
+  135 -
+  136 s
+  137 -
+  138 -
+  139 -
+  140 -
+  141 m
+  142 -
+  143 -
+  144 o
+  145 -
+  146 -
+  147 -
+  148 m
+  149 -
+  150 -
+  151 e
+  152 -
+  153 n
+  154 -
+  155 t
+  156 -
+  157 -
+  158 -
+  159 -
+  160 -
+  161 -
+  162 -
+  163 -
+  164 -
+  165 -
+  166 -
+  167 -
+  168 -
+
+To merge tokens, we use::
+
+  from icefall.ctc import merge_tokens
+  token_spans = merge_tokens(alignment)
+  for span in token_spans:
+    print(id2token[span.token], span.start, span.end)
+
+The output is given below::
+
+  i 32 33
+  h 35 37
+  a 37 38
+  d 41 42
+  t 44 45
+  h 45 46
+  a 47 48
+  t 50 51
+  c 54 55
+  u 58 60
+  r 63 64
+  i 65 66
+  o 72 73
+  s 79 80
+  i 83 84
+  t 85 86
+  y 88 89
+  b 93 94
+  e 95 96
+  s 101 102
+  i 110 111
+  d 113 114
+  e 114 115
+  m 116 117
+  e 119 120
+  a 124 125
+  t 127 128
+  t 129 130
+  h 130 131
+  i 132 133
+  s 136 137
+  m 141 142
+  o 144 145
+  m 148 149
+  e 151 152
+  n 153 154
+  t 155 156
+
+All of the code below is copied and modified
+from `<https://pytorch.org/audio/main/tutorials/ctc_forced_alignment_api_tutorial.html>`_.
+
+Segment each word using the computed alignments
+-----------------------------------------------
+
+.. code-block:: python3
+
+  def unflatten(list_, lengths):
+      assert len(list_) == sum(lengths)
+      i = 0
+      ret = []
+      for l in lengths:
+          ret.append(list_[i : i + l])
+          i += l
+      return ret
+
+
+  word_spans = unflatten(token_spans, [len(word) for word in transcript])
+  print(word_spans)
+
+The output is::
+
+  [[TokenSpan(token=2, start=32, end=33)],
+   [TokenSpan(token=15, start=35, end=37), TokenSpan(token=1, start=37, end=38), TokenSpan(token=13, start=41, end=42)],
+   [TokenSpan(token=7, start=44, end=45), TokenSpan(token=15, start=45, end=46), TokenSpan(token=1, start=47, end=48), TokenSpan(token=7, start=50, end=51)],
+   [TokenSpan(token=20, start=54, end=55), TokenSpan(token=6, start=58, end=60), TokenSpan(token=9, start=63, end=64), TokenSpan(token=2, start=65, end=66), TokenSpan(token=5, start=72, end=73), TokenSpan(token=8, start=79, end=80), TokenSpan(token=2, start=83, end=84), TokenSpan(token=7, start=85, end=86), TokenSpan(token=16, start=88, end=89)],
+   [TokenSpan(token=17, start=93, end=94), TokenSpan(token=3, start=95, end=96), TokenSpan(token=8, start=101, end=102), TokenSpan(token=2, start=110, end=111), TokenSpan(token=13, start=113, end=114), TokenSpan(token=3, start=114, end=115)],
+   [TokenSpan(token=10, start=116, end=117), TokenSpan(token=3, start=119, end=120)],
+   [TokenSpan(token=1, start=124, end=125), TokenSpan(token=7, start=127, end=128)],
+   [TokenSpan(token=7, start=129, end=130), TokenSpan(token=15, start=130, end=131), TokenSpan(token=2, start=132, end=133), TokenSpan(token=8, start=136, end=137)],
+   [TokenSpan(token=10, start=141, end=142), TokenSpan(token=5, start=144, end=145), TokenSpan(token=10, start=148, end=149), TokenSpan(token=3, start=151, end=152), TokenSpan(token=4, start=153, end=154), TokenSpan(token=7, start=155, end=156)]
+  ]
+
+
+.. code-block:: python3
+
+  def preview_word(waveform, spans, num_frames, transcript, sample_rate=bundle.sample_rate):
+      ratio = waveform.size(1) / num_frames
+      x0 = int(ratio * spans[0].start)
+      x1 = int(ratio * spans[-1].end)
+      print(f"{transcript} {x0 / sample_rate:.3f} - {x1 / sample_rate:.3f} sec")
+      segment = waveform[:, x0:x1]
+      return IPython.display.Audio(segment.numpy(), rate=sample_rate)
+  num_frames = emission.size(1)
+
+.. code-block:: python3
+
+   preview_word(waveform, word_spans[0], num_frames, transcript[0])
+   preview_word(waveform, word_spans[1], num_frames, transcript[1])
+   preview_word(waveform, word_spans[2], num_frames, transcript[2])
+   preview_word(waveform, word_spans[3], num_frames, transcript[3])
+   preview_word(waveform, word_spans[4], num_frames, transcript[4])
+   preview_word(waveform, word_spans[5], num_frames, transcript[5])
+   preview_word(waveform, word_spans[6], num_frames, transcript[6])
+   preview_word(waveform, word_spans[7], num_frames, transcript[7])
+   preview_word(waveform, word_spans[8], num_frames, transcript[8])
+
+The segmented wave of each word along with its time stamp is given below:
+
+.. raw:: html
+
+  <table>
+    <tr>
+      <th>Word</th>
+      <th>Time</th>
+      <th>Wave</th>
+    </tr>
+    <tr>
+      <td>i</td>
+      <td>0.644 - 0.664 sec</td>
+      <td>
+       <audio title="i.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/i.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>had</td>
+      <td>0.704 - 0.845 sec</td>
+      <td>
+       <audio title="had.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/had.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>that</td>
+      <td>0.885 - 1.026 sec</td>
+      <td>
+       <audio title="that.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/that.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>curiosity</td>
+      <td>1.086 - 1.790 sec</td>
+      <td>
+       <audio title="curiosity.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/curiosity.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>beside</td>
+      <td>1.871 - 2.314 sec</td>
+      <td>
+       <audio title="beside.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/beside.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>me</td>
+      <td>2.334 - 2.414 sec</td>
+      <td>
+       <audio title="me.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/me.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>at</td>
+      <td>2.495 - 2.575 sec</td>
+      <td>
+       <audio title="at.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/at.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>this</td>
+      <td>2.595 - 2.756 sec</td>
+      <td>
+       <audio title="this.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/this.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+    <tr>
+      <td>moment</td>
+      <td>2.837 - 3.138 sec</td>
+      <td>
+       <audio title="moment.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/moment.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+    </tr>
+  </table>
+
+We repost the whole wave below for ease of reference:
+
+.. raw:: html
+
+  <table>
+    <tr>
+      <th>Wave filename</th>
+      <th>Content</th>
+      <th>Text</th>
+    </tr>
+    <tr>
+      <td>Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav</td>
+      <td>
+       <audio title="Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav" controls="controls">
+             <source src="/icefall/_static/kaldi-align/Lab41-SRI-VOiCES-src-sp0307-ch127535-sg0042.wav" type="audio/wav">
+             Your browser does not support the <code>audio</code> element.
+       </audio>
+      </td>
+      <td>
+        i had that curiosity beside me at this moment
+      </td>
+    </tr>
+  </table>
+
+Summary
+-------
+
+Congratulations! You have succeeded in using the FST-based approach to
+compute alignment of a test wave.

docs/source/index.rst

@@ -25,7 +25,7 @@ speech recognition recipes using `k2 <https://github.com/k2-fsa/k2>`_.
    docker/index
    faqs
    model-export/index
-
+   fst-based-forced-alignment/index
 
 .. toctree::
    :maxdepth: 3
@@ -40,5 +40,5 @@ speech recognition recipes using `k2 <https://github.com/k2-fsa/k2>`_.
 
 .. toctree::
    :maxdepth: 2
-   
+
    decoding-with-langugage-models/index

docs/source/model-export/export-ncnn-conv-emformer.rst

@@ -15,8 +15,8 @@ We will show you step by step how to export it to `ncnn`_ and run it with `sherp
 
 .. caution::
 
-  Please use a more recent version of PyTorch. For instance, ``torch 1.8``
+  ``torch > 2.0`` may not work. If you get errors while building pnnx, please switch
-  may ``not`` work.
+  to ``torch < 2.0``.
 
 1. Download the pre-trained model
 ---------------------------------

docs/source/model-export/export-ncnn-lstm.rst

@@ -15,8 +15,8 @@ We will show you step by step how to export it to `ncnn`_ and run it with `sherp
 
 .. caution::
 
-  Please use a more recent version of PyTorch. For instance, ``torch 1.8``
+  ``torch > 2.0`` may not work. If you get errors while building pnnx, please switch
-  may ``not`` work.
+  to ``torch < 2.0``.
 
 1. Download the pre-trained model
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

docs/source/model-export/export-ncnn-zipformer.rst

@@ -15,8 +15,8 @@ We will show you step by step how to export it to `ncnn`_ and run it with `sherp
 
 .. caution::
 
-  Please use a more recent version of PyTorch. For instance, ``torch 1.8``
+  ``torch > 2.0`` may not work. If you get errors while building pnnx, please switch
-  may ``not`` work.
+  to ``torch < 2.0``.
 
 1. Download the pre-trained model
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^