Error if device mesh specified in fsdp config

[snarayan21]

What does this PR do?

Ensures that we throw value error if device mesh is directly specified in the fsdp config. We want users to more to the more descriptive, less confusing data_parallel_shard_degree and data_parallel_replicate_degree instead. Adding this error prevents users from accidentally configuring their parallelism differently than the device mesh would indicate -- we won't allow for directly specifying device mesh at all.

Basically changes the warning / warning test removed in #3570 to error / error test.

What issue(s) does this change relate to?

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[dakinggg]

this seems fine to me, so approving. But wondering if it would be a bit easier to make FSDPConfig take in a device_mesh arg and just raise if it is specified. That way you don't have to put the error in multiple places

[snarayan21]

@dakinggg I wanted to only error in the case where we used to throw the deprecation warning so this won't break users workflows without prior notice. For example, if someone explicitly creates an FSDPConfig object and passes that to the trainer, that wasn't marked as deprecated and we shouldn't error. We should only error for the deprecation path, which was for dict fsdp config + dict parallelism config (this covers foundry case). lmk if you disagree

taking out the error on _shard_with_fsdp, mistakenly left that in

[mvpatel2000]

We only want to set device mesh in state.py, so I agree with Daniel lets just check in constructor?

[snarayan21]

@mvpatel2000 made that change, modified init to still throw informative error for user