Convert string matches to enum comparison if possible

Created by: suchenzang

We have a lot of places where we do string matching, like:

https://github.com/facebookresearch/metaseq/blob/88ae968e679efbe84a8c246d1177852facfc43a2/metaseq/tasks/streaming_language_modeling.py#L339

or

https://github.com/facebookresearch/metaseq/blob/88ae968e679efbe84a8c246d1177852facfc43a2/metaseq_cli/train.py#L96

which can all be found via: ag "== \"" --py.

Convert these all to comparing against enums if possible.