add process_group in convert_sync_batchnorm (#19240)
Summary:
In line 508.
convert_sync_batchnorm is called recursively to convert the bn to syncbn, thus the process_group also should be passed in the function.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19240
Differential Revision: D15240318
Pulled By: ezyang
fbshipit-source-id: 0fc9e856392824814991e5e9e8f9513d57f311af
