What does this PR do?

Summary

Example: This PR adds CLARE attack, which uses distilled RoBERTa masked language model to perform word swaps, word insertions, word merges (which is where we combine two adjacent words and replace it with another word) in a greedy manner. s

Additions

• Example: Added clare recipe as textattack.attack_recipes.CLARE2020.

Changes

• Example: WordSwapMaskedLM has been updated to have a minimum confidence score cutoff and batching has been added for faster performance.

Deletions

• Example: Remove unnecessary files under textattack.models...

Checklist

• [ ] The title of your pull request should be a summary of its contribution.
• [ ] Please write detailed description of what parts have been newly added and what parts have been modified. Please also explain why certain changes were made.
• [ ] If your pull request addresses an issue, please mention the issue number in the pull request description to make sure they are linked (and people consulting the issue know you are working on it)
• [ ] To indicate a work in progress please mark it as a draft on Github.
• [ ] Make sure existing tests pass.
• [ ] Add relevant tests. No quality testing = no merge.
• [ ] All public methods must have informative docstrings that work nicely with sphinx. For new modules/files, please add/modify the appropriate .rst file in TextAttack/docs/apidoc.'