Add missing comments for beam decoding after refactoring of generate()
Author: ekagra-ranjan
Pull request: yes

# What does this PR do?

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This PR adds back some comments which helps in understanding the low level implementation details of beam decoding which was missed during an earlier refactoring of generate(). The comments were recovered from an [older state](https://github.com/yjernite/transformers/blob/356e825eeafb3539d7a1b332398511812602945f/src/transformers/generation_utils.py) of the beam decoding found using this [PR](https://github.com/huggingface/transformers/pull/5254/files#diff-b7601d397d5d60326ce61a9c91beaa2afa026014141052b32b07e1d044fbbe17).


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

@patrickvonplaten 

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Models:

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Comments:
_The documentation is not available anymore as the PR was closed or merged._