Adding the option to return_timestamps on pure CTC ASR models.
Author: Narsil
Pull request: yes

# What does this PR do?

Adding the option to return_timestamps on pure CTC ASR models.

This works by using the `output_char_offsets` during the decoding and
simply recovering the actual offset in seconds instead of number of tokens.

Works only on pure CTC right now.

Thanks @patrickvonplaten  for the earlier PR, made this one a breeze.

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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@patrickvonplaten , it seems `math.prod` is a Python 3.8 function, so the CI failed: https://app.circleci.com/pipelines/github/huggingface/transformers/34931/workflows/cf399730-6207-41bb-8c19-76f4b2036fe7/jobs/375024

I changed it but maybe I missed something to make it work.

> @patrickvonplaten , it seems `math.prod` is a Python 3.8 function, so the CI failed: https://app.circleci.com/pipelines/github/huggingface/transformers/34931/workflows/cf399730-6207-41bb-8c19-76f4b2036fe7/jobs/375024
> 
> I changed it but maybe I missed something to make it work.

Thanks for changing - LGTM!

@patrickvonplaten Do you think we can merge ?

Sorry checking now!

Thank you so much for the NITs !