What does this PR do?

Adds features

• Ability to specify device via TA_DEVICE env variable
• New constraint, MaxNumWordsModified
• Tracks previous AttackedText during attack to allow for reconstruction of chain of modifications
• ChangeGreedyWordSwapWIR to allow passing of specific unk token
• Format stuff
• Bump version to 0.3.5

Fixes bugs

• Downgrade USE from version 4 to 3 so that it works now (closes Tensorflow USE has been removed. #640)
• Update FLAIR usage for new API (closes ChecklistAugmenter AssertionError #638, AssertionError: Unexpected type of parameter 'use_tokenizer'. Parameter should be bool or Tokenizer #642)
• Formats things