[T5, generation] Add decoder caching for T5 (#3682)

* initial commit to add decoder caching for T5

* better naming for caching

* finish T5 decoder caching

* correct test

* added extensive past testing for T5

* clean files

* make tests cleaner

* improve docstring

* improve docstring

* better reorder cache

* make style

* Update src/transformers/modeling_t5.py

Co-Authored-By: Yacine Jernite <yjernite@users.noreply.github.com>

* make set output past work for all layers

* improve docstring

* improve docstring

Co-authored-by: Yacine Jernite <yjernite@users.noreply.github.com>

tests/test_modeling_common.py

@@ -128,6 +128,7 @@ class ModelTesterMixin:
         for model_class in self.all_model_classes:
             config.output_attentions = True
             config.output_hidden_states = False
+            config.output_past = False
             model = model_class(config)
             model.to(torch_device)
             model.eval()
@@ -144,10 +145,9 @@ class ModelTesterMixin:
             out_len = len(outputs)
 
             if self.is_encoder_decoder:
-                correct_outlen = (
-                    4  # decoder_features_or_logits, decoder_attentions, encoder_features, encoder_attentions
-                )
+                correct_outlen = 4
                 decoder_attention_idx = 1
+
                 if "lm_labels" in inputs_dict:  # loss will come first
                     correct_outlen += 1  # compute loss
                     decoder_attention_idx += 1