Update the installation doc, saying it requires at least k2 v1.7

docs/source/installation/images/k2-v-1.7.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="80" height="20" role="img" aria-label="k2: &gt;= v1.7"><title>k2: &gt;= v1.7</title><linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="r"><rect width="80" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#r)"><rect width="23" height="20" fill="#555"/><rect x="23" width="57" height="20" fill="blueviolet"/><rect width="80" height="20" fill="url(#s)"/></g><g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" text-rendering="geometricPrecision" font-size="110"><text aria-hidden="true" x="125" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="130">k2</text><text x="125" y="140" transform="scale(.1)" fill="#fff" textLength="130">k2</text><text aria-hidden="true" x="505" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="470">&gt;= v1.7</text><text x="505" y="140" transform="scale(.1)" fill="#fff" textLength="470">&gt;= v1.7</text></g></svg>

docs/source/installation/index.rst

@@ -7,6 +7,7 @@ Installation
 - |device|
 - |python_versions|
 - |torch_versions|
+- |k2_versions|
 
 .. |os| image:: ./images/os-Linux_macOS-ff69b4.svg
   :alt: Supported operating systems
@@ -20,7 +21,10 @@ Installation
 .. |torch_versions| image:: ./images/torch-1.6.0_1.7.0_1.7.1_1.8.0_1.8.1_1.9.0-green.svg
   :alt: Supported PyTorch versions
 
-icefall depends on `k2 <https://github.com/k2-fsa/k2>`_ and
+.. |k2_versions| image:: ./images/k2-v-1.7.svg
+  :alt: Supported k2 versions
+
+``icefall`` depends on `k2 <https://github.com/k2-fsa/k2>`_ and
 `lhotse <https://github.com/lhotse-speech/lhotse>`_.
 
 We recommend you to install ``k2`` first, as ``k2`` is bound to
@@ -32,12 +36,16 @@ installs its dependency PyTorch, which can be reused by ``lhotse``.
 --------------
 
 Please refer to `<https://k2.readthedocs.io/en/latest/installation/index.html>`_
-to install `k2`.
+to install ``k2``.
+
+.. CAUTION::
+
+  You need to install ``k2`` with a version at least **v1.7**.
 
 .. HINT::
 
   If you have already installed PyTorch and don't want to replace it,
-  please install a version of k2 that is compiled against the version
+  please install a version of ``k2`` that is compiled against the version
   of PyTorch you are using.
 
 (2) Install lhotse
@@ -50,10 +58,15 @@ to install ``lhotse``.
 
   Install ``lhotse`` also installs its dependency `torchaudio <https://github.com/pytorch/audio>`_.
 
+.. CAUTION::
+
+  If you have installed ``torchaudio``, please consider uninstalling it before
+  installing ``lhotse``. Otherwise, it may update your already installed PyTorch.
+
 (3) Download icefall
 --------------------
 
-icefall is a collection of Python scripts, so you don't need to install it
+``icefall`` is a collection of Python scripts, so you don't need to install it
 and we don't provide a ``setup.py`` to install it.
 
 What you need is to download it and set the environment variable ``PYTHONPATH``
@@ -367,7 +380,7 @@ Now let us run the training part:
 
 .. CAUTION::
 
-  We use ``export CUDA_VISIBLE_DEVICES=""`` so that icefall uses CPU
+  We use ``export CUDA_VISIBLE_DEVICES=""`` so that ``icefall`` uses CPU
   even if there are GPUs available.
 
 The training log is given below: