When on sagemaker use their env variables for saves
# What does this PR do?

When on SageMaker, the content of the env variable "SM_OUTPUT_DATA_DIR" should be used to save training artifacts (such as our checkpoint) so make it overwrite the `output_dir` (and make that argument optional so it doesn't need to be passed for sagemaker training).
Then the final model will be easy to deploy if it's also saved to the content of the env variable "SM_MODEL_DIR" so adding that as well.
