convert_sync_batchnorm should not convert _InstanceNorm instances (#29985)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/29187
This introduces a new class `_NormBase` that `_InstanceNorm` and `_BatchNorm` inherit from separately. This means the `isinstance(module, _BatchNorm)` check won't falsely pass for `_InstanceNorm`.
The suggested fix of adding `and not isinstance(module, _InstanceNorm)` works as well, but requires introducing a cyclic dependency between `instancenorm.py` and `batchnorm.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29985
Differential Revision: D18588104
Pulled By: yf225
fbshipit-source-id: f599da3b902ad9c56836db4d429bfc462ed51338
