Add a note about the CUDA OOM error.

Some users consider this kind of OOM as an error during decoding,
but actually it is not. This pull request clarifies that.

icefall/decode.py

@@ -732,6 +732,11 @@ def rescore_with_whole_lattice(
             logging.info(
                 f"num_arcs before pruning: {inv_lattice.arcs.num_elements()}"
             )
+            logging.info("This OOM is not an error. You can ignore it. "
+                    "If your model does not converge well, or --max-duration "
+                    "is too large, or the input sound file is difficult to "
+                    "decode, you will meet this exception."
+                    )
 
             # NOTE(fangjun): The choice of the threshold 1e-9 is arbitrary here
             # to avoid OOM. You may need to fine tune it.