[megatron] don't re-assert no_sync_func every step with overlap_grad_reduce

[HaozheZhang6]

Summary

train() in slime/backends/megatron_utils/model.py sets up config.no_sync_func on every step, but config = get_model_config(model[0]) is the model config and persists across steps. With --overlap-grad-reduce, the first step passes assert config.no_sync_func is None and sets it; the second step then trips that same assert and crashes.

assert config.no_sync_func is None, (...)   # step 1: None -> ok, sets it
                                            # step 2: not None -> AssertionError

Fix

Guard the setup with if config.no_sync_func is None: so the sync funcs are set once. They're constant (the model chunks' own no_sync / start_grad_sync), so skipping on later steps is a no-op — the only thing the per-step re-run did was trip the assert.

Reproduction

Run any training with --overlap-grad-reduce (from #1779) — it crashes on the 2nd step without this change.

Notes

• I couldn't add an automated test: reproducing needs a multi-GPU Megatron training run with --overlap-grad-reduce, which I can't stand up locally. The fix is small and verified by inspection against the reported repro; happy to add a test if you can point me at a lightweight harness for this path.
• Opening as a draft to confirm the approach: the original assert also doubled as a guard against a pre-supplied custom no_sync_func. Under the is None guard it becomes redundant, and a hypothetical pre-set custom func would now be left as-is rather than rejected. If you'd rather keep that rejection explicit (e.g. assert only on first setup), I'll adjust.

Fixes #1779