[Target] Add target tags for Apple Silicon GPU

As we are recently conducting experiments on GPU of Apple Silicon, we
find it helpful to introduce the target tag for Apple Silicon GPUs, as
specialized Metal targets.

At this moment, we find the size of shared memory per thread block, the
maximum number of threads per thread block and the warp size useful,
so each specialized chip is configured by these three parameters.

The feature numbers come from Apple's official document:
https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf

src/target/tag.cc

@@ -376,4 +376,19 @@ TVM_REGISTER_TAG_AWS_C5("aws/cpu/c5.24xlarge", 48, "cascadelake");
 
 #undef TVM_REGISTER_TAG_AWS_C5
 
+#define TVM_REGISTER_METAL_GPU_TAG(Name, ThreadsPerBlock, SharedMem, WarpSize)   \
+  TVM_REGISTER_TARGET_TAG(Name).set_config(                                      \
+      {{"kind", String("metal")},                                                \
+       {"max_threads_per_block", Integer(ThreadsPerBlock)},                      \
+       {"max_shared_memory_per_block", Integer(SharedMem)},                      \
+       {"thread_warp_size", Integer(WarpSize)},                                  \
+       {"host", Map<String, ObjectRef>{{"kind", String("llvm")},                 \
+                                       {"mtriple", String("arm64-apple-macos")}, \
+                                       {"mcpu", String("apple-latest")}}}});
+
+TVM_REGISTER_METAL_GPU_TAG("apple/m1-gpu", 1024, 32768, 32);
+TVM_REGISTER_METAL_GPU_TAG("apple/m2-gpu", 1024, 32768, 32);
+
+#undef TVM_REGISTER_METAL_TAG
+
 }  // namespace tvm