[Metal] Add concat op for metal
Summary: Add concat op to enable models such as SqueezeNet.
Test Plan:
Test on device:
```
arc focus2 pp-ios
```
Test on mac
```
buck test pp-macos
```
Reviewed By: xta0
Differential Revision: D26029029
fbshipit-source-id: b0d621f2069a722f0770218c435b22feac4fb873
