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* all save_pretrained methods mkdir if not os.path.exists

closes #4958

Co-authored-by: Daniel Shan <daniel.shan@workday.com>

* Kill model archive maps

* Fixup

* Also kill model_archive_map for MaskedBertPreTrainedModel

* Unhook config_archive_map

* Tokenizers: align with model id changes

* make style && make quality

* Fix CI

The option `--do_lower_case` is currently required by the uncased models (i.e., bert-base-uncased, bert-large-uncased).

Results:
BERT-BASE without --do_lower_case:  'exact': 73.83, 'f1': 82.22
BERT-BASE with --do_lower_case:  'exact': 81.02, 'f1': 88.34

* Add QA trainer example for TF

* Make data_dir optional

* Fix parameter logic

* Fix feature convert

* Update the READMEs to add the question-answering task

* Apply style

* Change 'sequence-classification' to 'text-classification' and prefix with 'eval' all the metric names

* Apply style

* Apply style

* Created using Colaboratory

* [examples] reorganize files

* remove run_tpu_glue.py as superseded by TPU support in Trainer

* Bugfix: int, not tuple

* move files around