[Bugfix] Fix the bug when the stop word is set to eos

[rucnyz]

The original _check_stop function truncates the generated sentences to prevent the output of user-set stop tokens. However, if the user sets stop_token_ids to tokenizer.eos_token_id in SamplingParams, by default, the eos_token would not appear in the generated sentences. Therefore, the _check_stop function would truncate the normal generated sentences, leading to a decrease in the quality of generation. After the fix, when the last word is eos, _check_stop will not truncate the sentence, ensuring normal generation performance.

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[njhill]

@rucnyz @simon-mo I'm not sure that this is the correct fix. When params.include_stop_str_in_output is False and params.skip_special_tokens is False, then you do want to truncate the eos token. When params.skip_special_tokens is true then no tuncation should be done for eos (because the token's text won't be in the output text), but it also shouldn't be done if the user-provided stop token is some other non-eos special token.

I am working on fixing all of this more comprehensively, along with some adjacent bugs in stop string handling (including #3572, #3574).

An immediate alternative to this fix, which I think would make sense, is to just move the existing eos check from the bottom to the top of the _check_stop method.

[rucnyz]

@njhill Thank you for your suggestion! I modified the code based on your alternative, so that at least we fix this issue with the current code. Looking forward to your comprehensive PR:)

[njhill]

@rucnyz this should be addressed by #3672

[njhill]

@rucnyz closing this now since the issue should be resolved by #3672. Please feel free to reopen another PR if you still don't see expected behaviour. Thanks for the contribution.