Mark test_rloo[fsdp2] as xfail for transformers 5.4.0

[albertvillanova]

Fix CI failure for distributed test_rloo[fsdp2]:

• Mark test_rloo[fsdp2] as xfail for transformers 5.4.0

Fix #5386.

See upstream fix:

• Fix NaN weights on non-rank-0 FSDP processes transformers#45050

This PR marks the "fsdp2" configuration of the distributed test suite as an expected failure (xfail) for a specific version of the transformers library, providing clearer test outcomes and documentation for known upstream issues.

Testing improvements:

• Added a new pytest.param for the "fsdp2" configuration in test_rloo, marking it as an expected failure (xfail) when transformers version is exactly 5.4.0, with a detailed reason and strict enforcement. This documents and accounts for an upstream issue causing NaN weights on non-rank-0 FSDP processes.

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Note

Low Risk
Low risk: test-only change that marks a known upstream transformers==5.4.0 FSDP issue as an expected failure, without affecting runtime code paths.

Overview
Marks the distributed test_rloo case for the fsdp2 configuration as xfail specifically on transformers==5.4.0, with a documented reason and strict=True to prevent silent passes.

This stabilizes CI by treating the known upstream NaN-weight failure on non-rank-0 FSDP processes as an expected, version-gated test outcome.

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