`GPT2Model` StaticCache support (#35761)
* initial GPT2 changes
* causal_mask support
* return_legacy_cache
* cleanup
* fix1
* outputs shape fixes
* gpt2 return fix
* pkv, attn fixes
* fix dual_head
* is_causal arg fix
* decision transformer updated
* style fix
* batch_size from inputs_embeds
* DecisionTransformerModel fixes
* cross-attn support + cache warning
* x-attn @decision
* EDCache proper init
* simplified logic in `if use_cache:` for GPT2Model
* @deprecate_kwarg for DecisionTr attn fwd
* @deprecate_kwarg in gpt2
* deprecation version updated to 4.51
* kwargs in gradient_checkpointing_fn
* rename next_cache to past_key_values
* attention_mask prep
* +cache_position in GPT2DoubleHeadsModel
* undo kwargs in gradient checkpointing
* moved up `if self.gradient_checkpointing`
* consistency in decision_transformer
* pastkv, cache_pos in grad_checkpt args
* rm _reorder_cache
* output_attentions streamlined
* decision_transformer consistency
* return_legacy_cache improved
* ClvpForCausalLM used for legacy cache test now
* is_causal fixed
* attn_output cleanup
* consistency @ decision_transformer
* Updated deprecation notice version to 4.52
* upd deprecation
* consistent legacy cache code in decision transformers\
* next_cache -> past_kv in decision_tr
* cache support flags in decision_transf
* rm legacy cache warning
* consistency in cache init for decision transf
* no Static Cache for Decision Transformer
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Co-authored-by: Cyril Vallez <cyril.vallez@huggingface.co>
