Add generate() functionality to TF 2.0 (#3063)

* add first copy past test to tf 2 generate

* add tf top_k_top_p_filter fn

* add generate function for TF

* add generate function for TF

* implemented generate for all models expect transfoXL

* implemented generate for all models expect transfoXL

* implemented generate for all models expect transfoXL

* make style

* change permission of test file to correct ones

* delete ipdb

* delete ipdb

* fix bug and finish simple gpt2 integration test

* clean test file

* clean test file

* make style

* make style

* make style

* make style

* change import style

* change import style

* make style

* make style

* add decorators

* add decorators

* fix tf ctrl bug dim => axis in TF

* make style

* make style

* refactored test file

* refactored test file

* take out test_torch_tf_conversion if nothing is defined

* take out test_torch_tf_conversion if nothing is defined

* remove useless files

* remove useless files

* fix conflicts

* fix conflicts

* fix conflicts

* fix conflicts

* fix conflicts

* solve conflicts

* solve conflicts

* fix conflicts

* fix conflicts

* merge conflicts

* delete ipdb

* exposed top_k_top_p_filtering fns

* delete weirdly created w! file

* add comment to test tf common modeling

* fix conflicts

* fix conflicts

* make style

* merge conflicts

* make style

* change tf.tensor.shape to shape_list(tensor)

tests/test_modeling_tf_openai_gpt.py

@@ -39,6 +39,9 @@ class TFOpenAIGPTModelTest(TFModelTesterMixin, unittest.TestCase):
     all_model_classes = (
         (TFOpenAIGPTModel, TFOpenAIGPTLMHeadModel, TFOpenAIGPTDoubleHeadsModel) if is_tf_available() else ()
     )
+    all_generative_model_classes = (
+        (TFOpenAIGPTLMHeadModel,) if is_tf_available() else ()
+    )  # TODO (PVP): Add Double HeadsModel when generate() function is changed accordingly
 
     class TFOpenAIGPTModelTester(object):
         def __init__(