Question regarding SubBatchNorm3d

Hello, thanks for sharing the code!

I have a question regarding the line:
https://github.com/facebookresearch/SlowFast/blob/9bcc68fed2a746973f0e155cc41ebb9c43561c6e/slowfast/models/batchnorm_helper.py#L78-L81

1. This is minor but "std" technically means variance right?
2. I was curious why you add an additional term after "stds.view(n, -1).sum(0) / n" when aggregating for std. Why it is not simply the average of sub batch statistics like you do for mean?

Thank you,
Jaejin Cho