[metal] Use MPSCNN kernels for binary elementwise ops
Summary: Previously, binary elementwise kernels such as add, sub, and mul were implemented with custom shaders. However, MPSCNN has kernels for these operations for iOS >=11.3. Update these ops to use MPSCNN kernels instead of shader implementations.
Test Plan:
Test on device:
`arc focus2 pp-ios`
Test on mac
`buck test pp-macos`
Reviewed By: xta0
Differential Revision: D25953986
fbshipit-source-id: 3acac3fa7dbe70f92572c21c0f0cfcdedfcdcf23
