[SR] NNC out variant for aten::where (#67255)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67255
Add an out variant for `aten::where`.
Since this op can be implemented quite trivially in NNC with `ifThenElse`, I added an NNC kernel as well.
Test Plan: Unit tests: `buck test caffe2/benchmarks/static_runtime:static_runtime_cpptest`
Reviewed By: navahgar
Differential Revision: D31923886
fbshipit-source-id: b4379ee3aaf31a000e626b4caeafd3e3f3d60837
