fix: skip FSDP DTensor boundary validation under fake process group

[Victarry]

Mirror of #3668
The validate_uneven_dtensor function uses all_reduce(MAX) across all ranks to verify that local shards collectively cover the full global tensor. Under fake process group (backend='fake'), all collective operations are no-ops, so only rank 0's boundaries are visible — the end-boundary check always fails.

Skip the boundary validation when the distributed backend is 'fake', since fake process group is only used for memory profiling where numerical correctness is irrelevant.

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[cspades]

LGTM, is this the only modification we need to make to Megatron-FSDP to support a mock backend across all cases? Now that I think about it, we there aren't many places we validate the direct output of a collective...

[Victarry]

LGTM, is this the only modification we need to make to Megatron-FSDP to support a mock backend across all cases? Now that I think about it, we there aren't many places we validate the direct output of a collective...

Yeah. I verified the FSDP with fake process group could work with this fix,.

[Victarry]

/ok to test 0f85443

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