fix: handle utf-8 decoding exceptions while processing std streams

Issue #, if available: During training, if the user script tries to write non utf-8 characters to Cloudwatch, an error is thrown because decoding fails and is not handled anywhere. This causes training to fail.

Description of changes: Adding a parameter to replace unhandled characters with a ? during utf-8 decoding. use � (U+FFFD, the official REPLACEMENT CHARACTER) for replacing. Refer: https://docs.python.org/3.9/library/codecs.html#error-handlers

Testing done:

Added unit test
Successful runs on SageMaker after fix

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aws / sagemaker-training-toolkit