convert DMPCollectionConfig and DMPCollectionContext to proper dataclasses

[iamzainhuda]

Summary: The DMPCollectionConfig and DMPCollectionContext classes used field() syntax at class level but were not decorated with dataclass. This meant:

1. The field() calls were being executed at class definition time but not processed by the dataclass machinery, leaving weird Field objects as default values
2. The __post_init__ method in DMPCollectionConfig was never being called, so the validation for ShardedModule was not working
3. The duplicate sharding_strategy field in DMPCollectionContext with init=False was overriding the parent's definition incorrectly

Differential Revision: D88991073

[meta-codesync[bot]]

@iamzainhuda has exported this pull request. If you are a Meta employee, you can view the originating Diff in D88991073.