[GPU] Add metal to DispatchKeySet (#46455)

Summary:
Pull Request resolved: #46455

After this PR(#46236 ) landed, the `aten::copy_` can no longer be dispatched to Metal kernels.
ghstack-source-id: 114499399

Test Plan:
- Sandcastle CI
- Circle CI

Reviewed By: IvanKobzarev, ailzhang

Differential Revision: D24356769

fbshipit-source-id: 8660ca5be663fdc8985d9eb710ddaadbb43b0ddd

c10/core/DispatchKeySet.h

@@ -204,6 +204,7 @@ constexpr DispatchKeySet autogradother_backends = DispatchKeySet({
   DispatchKey::FPGA,
   DispatchKey::MSNPU,
   DispatchKey::Vulkan,
+  DispatchKey::Metal,
   DispatchKey::MKLDNN,
   DispatchKey::OpenGL,
   DispatchKey::OpenCL,