Simplify configs and remove args scattering throughout

Created by: suchenzang

Before we can move to cleaner configs, we need to reduce the amount of args bloat in the codebase. This is an issue to track cleanup efforts on this front. Previous PRs include:

• https://github.com/facebookresearch/metaseq/pull/96
• https://github.com/facebookresearch/metaseq/pull/251
• https://github.com/facebookresearch/metaseq/pull/252
• https://github.com/facebookresearch/metaseq/pull/322
• https://github.com/facebookresearch/metaseq/pull/338
• https://github.com/facebookresearch/metaseq/pull/367