Add a cli tool to test nn training functionality

[thodkatz]

This PR adds a command line tool to test the functionality of the neural network training. I have included a README.md for its usage.

I wasn't really sure where to add it, I have created a seperate dir nn_training_cli, under the tiktorch/dev.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.40%. Comparing base (15a0225) to head (2f61bce).

Additional details and impacted files

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  Coverage   62.40%   62.40%           
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  Files          47       47           
  Lines        2878     2878           
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  Hits         1796     1796           
  Misses       1082     1082           

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