Backend init configs

[lucylq]

Provide configs to the backend during module init. These configs are local to the method instance, not global backend configurations like what backend options provide today.

Option 1: Extend BackendInitContext

• Pros; it's attached to the method and passed at init time.
• Cons; as a user, it can be confusing to understand what belongs in BackendInitContext vs BackendOptions vs NamedDataMap

Option 2: Backend Options

• Hacky, as these are global backend configs, not attached to a specific method.

Option 3: NamedDataMap

• Hacky, as NamedDataMap is intended for read-only data (like tensors or data blobs). Overloading it with configs is confusing, especially if it's already being used for the intended purpose.

More discussion here: https://docs.google.com/document/d/1JBNTSBfIHgWk979vtleby67z7WNUN-ClqMEJfMdeczY/edit?tab=t.0

And here: https://fb.workplace.com/groups/pytorch.edge.users/permalink/1957044708499146/

cc @metascroy