[Bugfix] Fix MultiprocessingGPUExecutor.check_health when world_size == 1

[jsato8094]

MultiprocessingGPUExecutor and MultiprocessingGPUExecutorAsync do not have the worker_monitor attribute when world_size == 1. However, the check_health() method requires this attribute, so it should be ignored in this case.

FIX #xxxx (link existing issues this PR will resolve)

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[njhill]

Thanks @jsato8094. I don't think it's expected that MultiprocessingGPUExecutor would be used with world_size == 1, it would make more sense to just use GPUExecutor.

But I think this fix is fine since it should currently otherwise work with world_size == 1 in theory.

It would be better though I think to set self.worker_monitor = None in the world_size == 1 case in _init_executor() and then the check here should only need to be if self.worker_monitor is not None and ... rather than checking for the attribute existence.

getattr is used in the shutdown method because this is called from the destructor and that can happen before the _init_executor() method completed successfully.

[jsato8094]

@njhill Thank you for your comment. Indeed, MultiprocessingGPUExecutor should not be used when world_size == 1. I encountered this issue because I mistakenly set engine_args.distributed_executor_backend to mp. Your idea to avoid using getattr is excellent, so I will implement this change.

[vrdn-23]

Nice work @jsato8094 @njhill ! Is this good to merge now?