Fix TF Trainer loss calculation (#6998)
Fix TF Trainer loss calculation (#6998)

* create branch for issue #6968

* First attempt to fix incorrect tf trainer loss calculation

* Fix training loss in metric

* fix tf trainer evaluation loss

* apply count_instances_in_batch() for eval and test datasets

* prototype of using a new argument in trainer_tf.py to fix loss issue

* some renaming and fix, in particular for evaluation methods

* fix bugs to have a running version

* change to @staticmethod

* apply style
huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers