Fix type annotation in `_ConvNd` for in_channels (#84302)
`_ConvNd` has an attribute `in_channels` that was mistakenly annotated as `_in_channels`.
This fixes https://github.com/pytorch/pytorch/issues/84223
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84302
Approved by: https://github.com/albanD
