Reducing memory usage: removing useless logits computation in generate() (#31292)

* Add .float() in all generation methods logit outputs

* Switch float-casting of logits to training only for main models

* Add `num_logits_to_keep` in Llama and add it by default in generate

* Apply style

* Add num_logits_to_keep as arg in prepare_input_for_generation

* Add support for Mistral

* Revert models except llama and mistral

* Fix default None value in _supports_num_logits_to_keep()

* Fix dimension of dummy input

* Add exception for prophetnet in _supports_num_logits_to_keep()

* Update _supports_num_logits_to_keep() to use inspect.signature()

* Add deprecation cycle + remove modification with pretraining_tp

* Apply style

* Add most used models

* Apply style

* Make `num_logits_to_keep` an int in all cases to remove if-else clause

* Add compile check for the warning

* Fix torch versions

* style

* Add gemma2

* Update warning version

* Add comment about .float operations in generation utils

* Add tests in GenerationTesterMixin and ModelTesterMixin

* Fix batch size for assisted decoding in tests

* fix small issues in test

* refacor test

* fix slicing removing dim issue

* Add nemotron support (should fix check-copy issue in CIs)

* Trigger new CIs

* Trigger new CIs

* Bump version

* Bump version in TODO

* Trigger CIs

* remove blank space

* Trigger CIs