[Config, Caching] Remove output_past everywhere and replace by use_cache argument (#3734)

* remove output_past from pt

* make style

* add optional input length for gpt2

* add use cache to prepare input

* save memory in gpt2

* correct gpt2 test inputs

* make past input optional for gpt2

* finish use_cache for all models

* make style

* delete modeling_gpt2 change in test file

* correct docstring

* correct is true statements for gpt2

tests/test_modeling_tf_common.py

@@ -460,10 +460,6 @@ class TFModelTesterMixin:
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
         input_ids = inputs_dict["input_ids"] if "input_ids" in inputs_dict else inputs_dict["inputs"]
 
-        if self.is_encoder_decoder:
-            # needed for Bart beam search
-            config.output_past = True
-
         for model_class in self.all_generative_model_classes:
             model = model_class(config)