Minor ScanKernels.cu cleanup (#65350)
Summary:
- Replace THCNumerics with `at::_isnan`
- Replace `contiguous` with `expect_contiguous`
- Don't use `contiguous` on output tensors. Instead skip the copy and
just create a new empty tensor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65350
Reviewed By: ezyang
Differential Revision: D31103501
Pulled By: ngimel
fbshipit-source-id: 9030869e28d6c570fad074fd0502076de8e2ab09
