ctc_loss: fix backward when 2d target tensor is larger than max_target_length (#20971)
Summary:
Previously, we didn't work when 2d target tensors had extra columns at the end. Now we just ignore those.
Also fix the confusion in the doc example regarding the number of classes.
Thank you, ypw-rich for the report with reproducing example.
Fixes: #20522
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20971
Differential Revision: D15535481
Pulled By: ezyang
fbshipit-source-id: 397e44e20165fc4fa2547bee9390d4c0b688df93
