[Metal] Fix GetFunction of metal runtime

This PR fixes a bug in `MetalModuleNode::GetFunction`.

The lambda passed to autoreleasepool is supposed to return
nothing. Prior to this PR, it returns a function at an early
exit branch, which may cause the error of `EXC_BREAKPOINT`
in Metal.

Author: MasterJH5574

CMakeLists.txt

@@ -520,6 +520,7 @@ if(BUILD_STATIC_RUNTIME)
   add_library(tvm_runtime STATIC
     $<TARGET_OBJECTS:tvm_runtime_objs>
     $<TARGET_OBJECTS:tvm_libinfo_objs>
+    $<TARGET_OBJECTS:tvm_ffi_objs>
     ${TVM_RUNTIME_EXT_OBJS}
   )
   set(NOTICE_MULTILINE

ffi/cmake/Utils/Library.cmake

@@ -16,7 +16,7 @@
 # under the License.
 function(add_dsymutil target_name)
   # running dsymutil on macos to generate debugging symbols for backtraces
-  if(APPLE)
+  if(APPLE AND TVM_FFI_USE_LIBBACKTRACE)
     find_program(DSYMUTIL dsymutil)
     mark_as_advanced(DSYMUTIL)
     add_custom_command(TARGET ${target_name}

src/runtime/metal/metal_module.mm

@@ -267,7 +267,7 @@ void operator()(ffi::PackedArgs args, ffi::Any* rv, const ArgUnion64* pack_args)
     auto it = fmap_.find(name);
     if (it == fmap_.end()) {
       ret = ffi::Function();
-      return ret;
+      return;
     }
     const FunctionInfo& info = it->second;
     MetalWrappedFunc f;