Removes overwrites for output_dir 
# 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.