[generate] deprecate forced ids processor

[sanchit-gandhi]

What does this PR do?

First stage of deprecation for the ForceTokensLogitsProcessor. This logits processor was exclusively used for Whisper. However, the PR #29485 removed the last calls to this processor from Whisper, in favour of decoder_input_ids set internally by generate, meaning this processor can now be removed.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[ArthurZucker]

Glad that we removed it!

[ArthurZucker]

we should probably use logger.warning_once()?

[sanchit-gandhi]

This argument is to distinguish between when the user has passed forced_ids to generate, in which case we throw a warning explaining so:

transformers/src/transformers/generation/utils.py

Lines 921 to 926 in 4a604ed

	if generation_config.forced_decoder_ids is not None:
	# TODO(Sanchit): deprecate in v4.40 by removing this logic
	warnings.warn(
	"You have explicitly specified`forced_decoder_ids`. This functionality has been deprecated and will throw an error in v4.40. Please use `input_ids` or `decoder_input_ids` respectively.",
	FutureWarning,
	)

OR if the user instantiates the forced logits processor standalone. Again, we give a descriptive warning explaining this:

transformers/src/transformers/generation/logits_process.py

Lines 1751 to 1756 in 4a604ed

	if not _has_warned:
	# TODO(Sanchit): remove this processor entirely in v4.40
	warnings.warn(
	"This `ForceTokensLogitsProcessor` has been deprecated and will be removed in v4.40. Please pass `input_ids` or `decoder_input_ids` to the generate method directly.",
	FutureWarning,
	)

=> we don't want to trigger the second warnings if the user does option 1, since the processor has been instantiated automatically. Thus, we have a separate warning for each case. This should not be confused with an argument to detect whether the warning has previously been triggered