BatchNorm: fix mixed precision usage with affine=False (#61962)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/61924
The fused backward kernel was using the weight dtype to detect mixed precision usage, but the weights can be none and the `running_mean` and `running_var` can still be mixed precision. So, I update the check to look at those variables as well.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61962
Reviewed By: albanD
Differential Revision: D29825516
Pulled By: ngimel
fbshipit-source-id: d087fbf3bed1762770cac46c0dcec30c03a86fda
