Update documentation for CTCLoss (#18415)

Update documentation for CTCLoss (#18415)

Summary:
This is meant to resolve #18249, where I pointed out a few things that could improve the CTCLoss docs.

My main goal was to clarify:
- Target sequences are sequences of class indices, excluding the blank index
- Lengths of `target` and `input` are needed for masking unequal length sequences, and do not necessarily = S, which is the length of the longest sequence in the batch.

I thought about Thomas's suggestion to link the distill.pub article, but I'm not sure about it. I think that should be up to y'all to decide.

I have no experience with .rst, so it might not render as expected :)

t-vi ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18415

Differential Revision: D14691969

Pulled By: soumith

fbshipit-source-id: 381a2d52307174661c58053ae9dfae6e40cbfd46