Update ModelTrainer to support s3 uri and tar.gz file as source_dir

[mollyheamazon]

Issue #, if available:

Description of changes:

Testing done:

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• I have read the CONTRIBUTING doc
• I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the Python SDK team
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[benieric]

LGTM, just needs some tests