[build] Warning Suppression PR #4: Fixed warnings with MacOS (#4926)

* [build] Warning Suppression PR #1: Turned on -Wno-ignored-attributes & Removed unused functions

* [build] Warning Suppression PR #2: Eliminate warnings from third-party headers

* Fixed an warning with enum comparison

* [build] Warning Suppression PR #4: Fixed Mac-specific warnings

taichi/backends/metal/codegen_metal.cpp

@@ -190,7 +190,7 @@ class KernelCodegenImpl : public IRVisitor {
 
     for (int i = 0; i < compiled_snode_trees_.size(); ++i) {
       const auto &cst = compiled_snode_trees_[i];
-      for (const auto [node_id, _] : cst.snode_descriptors) {
+      for (const auto &[node_id, _] : cst.snode_descriptors) {
         RootInfo ri{};
         ri.snode_id = cst.root_id;
         ri.index_in_cst = i;

taichi/backends/metal/device.cpp

@@ -245,13 +245,20 @@ class StreamImpl : public Stream {
       const std::vector<StreamSemaphore> &wait_semaphores) override {
     auto *cb = static_cast<CommandListImpl *>(cmdlist)->command_buffer();
     commit_command_buffer(cb);
+
+    // FIXME: Implement semaphore mechanism for Metal backend
+    //        and return the actual semaphore corresponding to the submitted
+    //        cmds.
+    return nullptr;
   }
   StreamSemaphore submit_synced(
       CommandList *cmdlist,
       const std::vector<StreamSemaphore> &wait_semaphores) override {
     auto *cb = static_cast<CommandListImpl *>(cmdlist)->command_buffer();
     commit_command_buffer(cb);
     wait_until_completed(cb);
+
+    return nullptr;
   }
 
   void command_sync() override {

taichi/backends/metal/kernel_manager.cpp

@@ -188,7 +188,7 @@ class UserMtlKernel : public CompiledMtlKernelBase {
     // 0 is valid for |num_threads|!
     TI_ASSERT(kernel_attribs_.advisory_total_num_threads >= 0);
     BindBuffers buffers;
-    for (const auto b : kernel_attribs_.buffers) {
+    for (const auto &b : kernel_attribs_.buffers) {
       buffers.push_back({input_buffers.find(b)->second, b});
     }
     launch_if_not_empty(std::move(buffers), command_buffer);
@@ -215,7 +215,7 @@ class SparseRuntimeMtlKernelBase : public CompiledMtlKernelBase {
   void launch(InputBuffersMap &input_buffers,
               MTLCommandBuffer *command_buffer) override {
     BindBuffers buffers;
-    for (const auto b : kernel_attribs_.buffers) {
+    for (const auto &b : kernel_attribs_.buffers) {
       if (b.type() == BufferDescriptor::Type::Context) {
         buffers.push_back({args_buffer_.get(), b});
       } else {

taichi/backends/metal/metal_program.cpp

@@ -11,7 +11,7 @@ namespace {
 std::unordered_set<const SNode *> find_all_dense_snodes(
     const metal::SNodeDescriptorsMap &snodes_map) {
   std::unordered_set<const SNode *> res;
-  for (const auto [_, desc] : snodes_map) {
+  for (const auto &[_, desc] : snodes_map) {
     const auto *sn = desc.snode;
     if (sn->type == SNodeType::dense) {
       res.insert(sn);

taichi/backends/opengl/codegen_opengl.cpp

@@ -224,7 +224,7 @@ class KernelGen : public IRVisitor {
         kernel_header += shaders::kOpenGlAtomicF32Source_gtmp;
       }
       std::unordered_set<int> arr_ids;
-      for ([[maybe_unused]] const auto [arr_id, bind_idx] :
+      for ([[maybe_unused]] const auto &[arr_id, bind_idx] :
            used.arr_arg_to_bind_idx) {
         arr_ids.insert(arr_id);
       }