[meta issue] Refactoring Efforts

[ashors1]

This issue tracks the refactoring efforts across Megatron and DTensor backends.

DTensor refactor: #1589
Megatron refactor: #1593

Each backend will have three corresponding refactoring PRs:

1. Refactor init and setup
2. Refactor data utilities
3. Refactor train/forward utilities

Please refer to the backend-specific issues for the links to the PRs. PRs should be merged sequentially for each backend: (1) --> (2) --> (3)