Removes overwrites for output_dir

[philschmid]

What does this PR do?

This PR removes the overwrites of the output_dir when running training on SageMaker and it removes the automatic save if output_dir is None.
The overwrites have been removed since it prevents saving checkpoints to a different dir like opt/ml/checkpoints impossible and it is not the "transformers" way. We can keep it the same as running training somewhere else and provide documentation on what you need to do.

[sgugger]

Thanks for doing this!
Since this PR removes the auto-value for output_dir in the only situation it was optional, it should also remove the default in training_args (the argument is not optional anymore).