tests: fix unreachable DeepSpeed+cpu_offload xfail in test_training_feature_matrix

[UgaTheDev]

In tests/smoke/test_train.py::test_training_feature_matrix, the DeepSpeed
branch was:

else:
    pytest.xfail("DeepSpeed not currently functional. OOMs during backprop.")
    if cpu_offload:
        pytest.xfail("DeepSpeed CPU Adam isn't currently building correctly")
    train_args.deepspeed_options.cpu_offload_optimizer = cpu_offload

pytest.xfail() raises immediately, so the if cpu_offload: block and the
cpu_offload_optimizer assignment below it could never run — both
DeepSpeed-parametrized cases (cpu_offload=True and cpu_offload=False)
always hit the same generic message and the more specific CPU-Adam reason was
dead code.

This reorders the checks so the cpu_offload=True case reports its own
specific, more accurate xfail reason, and only the cpu_offload=False case
falls through to the general "OOMs during backprop" one:

else:
    if cpu_offload:
        pytest.xfail("DeepSpeed CPU Adam isn't currently building correctly")
    pytest.xfail("DeepSpeed not currently functional. OOMs during backprop.")

Verified both xfail messages are now reachable by simulating the branch
logic standalone (can't run the real test without GPU hardware / the full
package installed):

cpu_offload=True: raised XFailed: DeepSpeed CPU Adam isn't currently building correctly
cpu_offload=False: raised XFailed: DeepSpeed not currently functional. OOMs during backprop.

Also confirmed the file still parses (python3 -m py_compile).

This is a test-only change — no non-test code touched, no new coverage
claimed, just makes the two existing xfail reasons distinguishable per the
docstring/#705 finding: "the second xfail below it for DeepSpeed plus CPU
offload... that block and the deepspeed_options.cpu_offload_optimizer
assignment after it are unreachable."

Originally flagged in #705 (comment from 2026-07-29).

[coderabbitai]

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Reviewing files that changed from the base of the PR and between da76404 and 9431891.

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• tests/smoke/test_train.py

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