Remove unreachable NCCL broadcast receiver code

[TianyeGGBond]

What changed

• Removed unreachable receiver-side NCCL broadcast fan-out code after the unconditional NotImplementedError in MegatronTrainRayActor._dispatch_nccl_broadcast.
• Kept the existing fail-fast guard so MILES still refuses NCCL broadcast transport until the sender-side NCCL path is implemented.
• Shortened the method docstring so the implementation explains the guard without carrying review-history details in code comments.

Why

The sender-side NCCL implementation is intentionally not available yet, so _dispatch_nccl_broadcast must fail before receiver engines enter collective setup and wait for a rank-0 sender that never joins. The old version correctly protected that path, but left about 50 lines of permanently unreachable receiver fan-out code after the raise. That dead code creates review and lint noise without changing behavior.

Validation

• python -m py_compile miles/backends/megatron_utils/actor.py
• git diff --check