Fix tensorflow_dataset glue support
Fix tensorflow_dataset glue support

`glue_convert_examples_to_features` assumed that tensorflow_dataset
examples contains the features `'sentence1'` and `'sentence2'`. This
commit encapsulates the choice of features in the glue processor and
uses that to parse examples.

huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers