StoppingCriteria tracks elements separately in the batch

[zucchini-nlp]

What does this PR do?

As was pointed out in #28932 , StoppingCriteria needs to stop generation per batch element and return a boolean tensor of batch_size. This PR adds the logic to track each row and when StoppingCriteria is triggered, stop generating for that particular row only.

Note that the when #28932 gets merged, we need to add logic to handle beam related generation. The problem is that beam search has an internal logic of tracking EOS tokens and adds candidate tokens to hypothesis when done. And if StoppingCriteria will take the responsibility to track custom EOS tokens, it has to be passed to beam scorer. Right now I am not sure if calling StoppingCriteria twice is a good decision. First time to check candidate beams, and the second time for the chosen beams. What do you think @gante? It can be something like:

cur_len = input_ids.shape[-1] + 1
beam_next_input_ids = torch.cat([input_ids[next_indices, :], next_tokens.unsqueeze(-1)], dim=-1)
beam_next_input_ids = beam_next_input_ids.view(-1, cur_len)
next_is_done = stopping_criteria(beam_next_input_ids, scores)

# is all are done, then it's prob max_length, not custom EOS being triggered
if all(next_is_done):
    next_is_done = torch.full_like(next_indices, False, dtype=torch.bool)
next_is_done = next_is_done.view(next_indices.shape)

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [X] Did you read the contributor guideline, Pull Request section?
• [X] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [X] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [X] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR. @gante

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