Dependency resolved and slight performance enhancements

[saadz-khan]

The issues with its current version:

• Broken code due to inappropriate requirements.txt
• Version conflicts between different packages
• Some unclear steps in the README on data-used and how to retrieve it.
• GPU usage only (Not everyone owns one)

This PR covers:

• Slight improvements for performance enhancements.
• Pinning requirements explicitly requirements.txt with all the proper versions so that code does not break if packages are updated in the future.
• Update README.md for proper and easy use of the repo.
• CPU made the default device with clear steps to change to GPU in the config files.
• data_convert.py example added with proper description and comments for .bin to .txt.src and .txt.tgt file conversion.
• Descriptive colab-notebook added for getting the data raw cnn-dm stories to .txt.src and .txt.tgt