[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

src/transformers/modeling_bart.py

@@ -933,7 +933,7 @@ class BartForConditionalGeneration(PretrainedBartModel):
 
         return outputs
 
-    def prepare_inputs_for_generation(self, decoder_input_ids, past, attention_mask, **kwargs):
+    def prepare_inputs_for_generation(self, decoder_input_ids, past, attention_mask, use_cache, **kwargs):
         assert past is not None, "past has to be defined for encoder_outputs"
 
         # first step, decoder_cached_states are empty
@@ -947,7 +947,7 @@ class BartForConditionalGeneration(PretrainedBartModel):
             "decoder_cached_states": decoder_cached_states,
             "decoder_input_ids": decoder_input_ids,
             "attention_mask": attention_mask,
-            "use_cache": True,  # change this to avoid caching (presumably for debugging)
+            "use_cache": use_cache,  # change this to avoid caching (presumably for debugging)
         }
 
     def prepare_scores_for_generation(self, scores, cur_len, max_length):
@@ -980,10 +980,6 @@ class BartForConditionalGeneration(PretrainedBartModel):
     def get_output_embeddings(self):
         return _make_linear_from_emb(self.model.shared)  # make it on the fly
 
-    def _do_output_past(self, *args, **kwargs):
-        """ We should always use the cache in generate."""
-        return True
-
 
 @add_start_docstrings(
     """Bart model with a sequence classification/head on top (a linear layer on top of the pooled output) e.g. for GLUE tasks. """,