Why gather if the results isn't used?

[jerome-white]

detectron2/detectron2/engine/train_loop.py

Lines 335 to 337 in 453d795

	all_metrics_dict = comm.gather(metrics_dict)
	if comm.is_main_process():

In this snippet, gather is run before deciding whether processing should take place. The alternative to this if-statement is an implicit method return -- nothing explicitly takes place with all_metrics_dict if that statement fails. I've seen this convention in some of the evaluators as well. Is there some side effect of comm.gather such that running it irrespective of being in the main process is required? Would the semantics of this method be the same if that statement was run inside the if-block? I've looked as deep as the PyTorch code that gets called, but things are still mysterious.

I'm building my own evaluators that may run in a distributed environment, so understanding the nuances of this feels important.