Update issues found in NVIDIA Transformer data processing scripts and make better instructions in README for data processing.

[kevinstephano]

1. The Transformer data processing scripts had logging imports that did not really exist and caused errors for users attempting to use the scripts.
2. Updated the download path for the test dataset as the original path disappeared
3. Made explicit instructions in the README.md on how to execute dataset downloading, tokenization, and conversion to the file format needed by pytorch.

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[kevinstephano]

@googlebot I signed it!

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