Add taskmaster2 command-line arg to specify subset of domains (#3135)
Add taskmaster2 command-line arg to specify subset of domains (#3135)

* Add taskmaster2 command-line arg for single domain

Before this change, `parlai dd -t taskmaster2 --display-verbose` displayed a bunch of sports conversations.

After this change, running `parlai dd -t taskmaster2 --display-verbose --domains music` displays music conversations. Tried on a few other domains to validate; also had a print in the `_load_data()` function.

Also verified that no argument case that all domains were used +
defining `domains` multiple times only used the last one.

* Add taskmaster2 command-line arg for single domain

Before this change, `parlai dd -t taskmaster2 --display-verbose`
displayed a bunch of sports conversations.

After this change, running `parlai dd -t taskmaster2
--display-verbose --domains music` displays a
music.

Also verified that no argument case that all domains were used.
facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI