Simplify MNIST and NLP challenge via AWS Open Data: Solution Branch

[rosalieandico]

Issue #, if available: Simplify MNIST and NLP challenge via AWS Open Data

Description of changes: Updated NLP/MNIST exercises.

• local_dir
• decoupled upload to S3 for simplicity and provided description - workshop feedback
• utilized --exclude and --include aws s3 sync options for better file/object management
• re-arranged role,session, bucket assignment immediately before use
• removed util.nb - not needed
• modified util.preprocessing

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