delete unimplemented broadcast transport path [need discussion]

[TianyeGGBond]

Context

MilesModelUpdateService.sync_selected_workers was named/documented as a
two-path weight transport (cpu_serialize + NCCL broadcast) and exposed a
public broadcast_local_ranks kwarg. But any non-empty value immediately
raised NotImplementedError: the cache-owner sender-side NCCL path
(init_process_group + per-bucket dist.broadcast) was never wired — only
the receiver-side fan-out existed.

So the only path that ever runs is cpu_serialize. The broadcast kwarg,
subset validation, NCCL rank assignment, and world_size math are dead
code that makes the surface look like it offers something it doesn't.

Change

pipeline/miles_model_update_service.py:

• Remove the broadcast_local_ranks kwarg from sync_selected_workers,
  along with the subset validation and the NotImplementedError branch.
  Every target now goes over cpu_serialize.
• Drop the broadcast_set parameter threaded through _run_atomic_unit
  and _build_plan, and the broadcast field from the done-log line.
• In _build_plan, replace the broadcast NCCL rank loop / world_size
  calculation with single-path constants.
• Tighten the docstring/comments to describe the single cpu_serialize path.

Wire compatibility

The wire plan shape is intentionally unchanged. broadcast_local_ranks is
still present (empty), comm_ranks are all 0, and world_size is 1 — the
exact values the cpu_serialize-only path already produced. The MILES
run_sync_session plan-key validator sees no difference.

Net: +20 / -60 lines, no behavior change on the live path.