Introduce save_strategy training argument
* Introduce save_strategy training argument

* collapse EvaluationStrategy and LoggingStrategy into a single TimeStrategy enum

* modify tests to use modified enum
# What does this PR do?
1. Introduce new `save_strategy` argument to decide on interval between 2 model saves during training.
2. Introduce a unified enum `TimeStrategy` which is used across `evaluation_strategy`, `logging_strategy` and `save_strategy`.

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@sgugger
Hi @sgugger,  
Got some time to raise the changes we talked about in [my previous PR](https://github.com/huggingface/transformers/pull/10267).  
Do let me know if I missed something.  
Thanks!<|||||>Hi @LysandreJik / @sgugger,

Thanks for your inputs! I can think of some better names, _but before that_:  
Is deprecating usage of `EvaluationStrategy` and keeping its definition along with the `TimeStrategy` (or whatever the name would be) in `trainer_utils.py` for the time being a good option? Can also throw a FutureWarning when `EvaluationStrategy` is used.<|||||>Yes that would be the preferred option: not use it anymore but still keep it until v5, and each time someone uses it a `FutureWarning` indicating it's deprecated and will be removed in version 5 is thrown.
Let me know if you have other questions!<|||||>Hi @sgugger,
Seems like the latest changes are failing a certain `make modified_only_fixup` test.
I'm not entirely sure where this test is failing.
Given that this test is passing on my local machine, this could be due to erratic hard-updation of some test/doc?

> 2021-02-26T19:45:22.6161202Z Checking/fixing src/transformers/__init__.py src/transformers/integrations.py src/transformers/models/__init__.py src/transformers/models/auto/configuration_auto.py src/transformers/models/auto/modeling_auto.py src/transformers/models/auto/modeling_tf_auto.py src/transformers/trainer_callback.py src/transformers/trainer_tf.py src/transformers/trainer_utils.py src/transformers/training_args.py src/transformers/training_args_tf.py src/transformers/utils/dummy_pt_objects.py src/transformers/utils/dummy_tf_objects.py src/transformers/utils/dummy_tokenizers_objects.py src/transformers/utils/notebook.py tests/test_trainer.py tests/test_trainer_callback.py utils/check_repo.py
> 2021-02-26T19:45:24.8027639Z All done! ✨ 🍰 ✨
> 2021-02-26T19:45:24.8028839Z 18 files left unchanged.
> 2021-02-26T19:45:28.4905837Z tests/test_trainer.py:1060:37: F821 undefined name 'EvaluationStrategy'
> 2021-02-26T19:45:28.5127548Z make: *** [modified_only_fixup] Error 1
> 2021-02-26T19:45:28.5129201Z Makefile:7: recipe for target 'modified_only_fixup' failed<|||||>Oh and for the failing test, you missed an `EvaluationStrategy` toward the end of `tests/test_trainer.py`, that's why you have the error.<|||||>Thanks! Fixed it now.
Although I see some approaches mentioned on other forums, I'm not entirely sure what would be the best approach to print a warning on usage of enum `EvaluationStrategy`.
If not too complex, you can point me towards how to do it.
Otherwise, you can merge this PR :-).<|||||>I'm not finding anything easy to do that, so I think we can merge for now and I'll keep looking.