Add FairScore Perturber to Model Zoo

[Rebecca-Qian]

Patch description This PR creates a fairscore entry in the model zoo for the demographic perturber model. The perturber is trained to rewrite text to change demographic references along gender, race and age axes.

Testing steps Tested downloads locally by serving model files on localhost, as well as downloads from S3. Example usage: parlai i -m bart -mf zoo:fairscore/model

Enter Your Message: Jane, man <PERT_SEP> Jane cooks dinner for her family every night.
[Bart]: John cooks dinner for his family every night.
Enter Your Message: [DONE]
CHAT DONE
... preparing new chat...
Enter Your Message: old, young <PERT_SEP> The old man tripped and fell, dropping his groceries.
[Bart]: The young man tripped and fell, dropping his groceries.

[github-actions[bot]]

This PR has not had activity in 30 days. Closing due to staleness.

[klshuster]

@Rebecca-Qian just flagging this is still open

[github-actions[bot]]

This PR has not had activity in 30 days. Closing due to staleness.