NLP text augmentation
This PR adds a text augmentation script. The script can be used to generate human-like typos when testing model robustness.

# Summary

The script supports the following text augmentations:

1. Dropping a letter in a word
2. Swapping two letters in a word
3. Human-like typos (based on keyboard structure, and common misspelling) using https://github.com/facebookresearch/AugLy package.

