transformers: Support returning raw logits in `generate`

Feature request

Support returning raw logits in generate by either:

1. creating a new arg that enables return of raw logits
2. or support callback that allow users to collect the raw logits

Motivation

• Raw logits “would be the most understandable & consistent across generation methods” (@patrickvonplaten)
• For testing, returning raw logits would help “identify which parts get wrong if any test failure occurs” (@ydshieh)
• There’s concern about “rampant too many options” (@Narsil), thus I would prefer the second option to support this feature.
• However, the second option still needs code change to support it. As the user provided logits_processor is appended to a new instance of LogitsProcessorList. As a result, users cannot get the raw logits using the current implementation even with a custom LogitsProcessor.

See further discussion in https://github.com/huggingface/transformers/issues/17424

Your contribution

I could open a PR to reorder how logits_processor is merged with the predefined list of LogitsProcessorList.