Add E2E API Integration Tests

Created by: suchenzang

API codepaths sometimes break with changes since they are not as frequently tested as our training codepaths. This has come up as an important need given ongoing efforts to clean up / refactor certain pieces of metaseq.

This can potentially help address:

• https://github.com/facebookresearch/metaseq/issues/385
• https://github.com/facebookresearch/metaseq/issues/383
• https://github.com/facebookresearch/metaseq/issues/23

And potentially relates to https://github.com/facebookresearch/metaseq/issues/268 as well.