Always reconnect rollout engines on weight-update setup

[fzyzcjy]

Unconditionally disconnect-then-reconnect the model-update process group when
(re)connecting rollout engines, guarding the destroy against a missing group, so
a reconfigured/healed engine set can rebuild the NCCL group from scratch.

• broadcast.py: drop the "only disconnect if group exists" short-circuit; guard
  destroy_process_group against None.

[gemini-code-assist]

Code Review

This pull request simplifies the conditional check in connect_rollout_engines by moving the None check for model_update_groups into disconnect_rollout_engines_from_distributed. The reviewer suggested wrapping the process group destruction in a try...finally block to ensure that the remote rollout engine destruction references are always awaited, even if destroying the local process group raises an exception.

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[gemini-code-assist]

If dist.destroy_process_group(model_update_groups) raises an exception (e.g., due to a timeout or NCCL error), the subsequent ray.get(refs) will be skipped. This can leave the rollout engines in an inconsistent state where they are still destroying the old process group while the training side attempts to recreate it, potentially leading to deadlocks or port conflicts during reconfiguration/healing.

Using a try...finally block ensures that we always await the remote destroy calls on the rollout engines, even if destroying the local process group fails.

Suggested change

	if model_update_groups is not None:
	dist.destroy_process_group(model_update_groups)
	ray.get(refs)
	try:
	if model_update_groups is not None:
	dist.destroy_process_group(model_update_groups)
	finally:
	ray.get(refs)

References

1. To prevent resource leaks (e.g., counters that are not decremented), use constructs like try...finally or a with statement to ensure cleanup logic is always executed, even in the case of exceptions or early returns.

[Shi-Dong]

I think the Gemini bot raised a valid concern. Please address.

[yueming-yuan]

I think the Gemini bot raised a valid concern. Please address.

hmm I think the concern will not happen given the overall design?