[Bug] convert_sync_batchnorm missing 'training' attribute

[collinmccarthy]

Prerequisite

• I have searched Issues and Discussions but cannot get the expected help.
• The bug has not been fixed in the latest version(https://github.com/open-mmlab/mmengine).

Environment

(Issue obvious from source)

Reproduces the problem - code sample

(Issue obvious from source)

Reproduces the problem - command or script

(Issue obvious from source)

Reproduces the problem - error message

(Issue obvious from source)

Additional information

In torch.nn.modules.batchnorm.py the SyncBatchNorm.convert_sync_batchnorm() method copies over the training attribute like module_output.training = module.training.

The mmengine version is missing this. However it is present in the revert_sync_batchnorm() method right above.

Not having this will cause a NCCL timeout when the BN layer is kept in eval mode for fine-tuning while the model is in training mode. I figured this out due to a similar issue/solution here