[WebNN EP] Decompose Concat with input number > 4 for CPU backend (mi…

…crosoft#18930)

WebNN XNNPack backend only supports the concat with inputs number <= 4,
decomposing the Concat with inputs number > 4 into multiple WebNN concat
ops.

onnxruntime/core/providers/webnn/builders/impl/concat_op_builder.cc

@@ -36,13 +36,40 @@ Status ConcatOpBuilder::AddToModelBuilderImpl(ModelBuilder& model_builder,
   NodeAttrHelper helper(node);
   uint32_t axis = static_cast<uint32_t>(HandleNegativeAxis(helper.Get("axis", 1), rank));
 
-  emscripten::val inputs = emscripten::val::array();
-  for (const auto* input : node.InputDefs()) {
+  const size_t num_inputs = input_defs.size();
+  std::vector<emscripten::val> inputs;
+  for (const auto* input : input_defs) {
     LOGS(logger, VERBOSE) << "input name " << input->Name();
-    inputs.call<void>("push", model_builder.GetOperand(input->Name()));
+    inputs.push_back(model_builder.GetOperand(input->Name()));
   }
 
-  emscripten::val output = model_builder.GetBuilder().call<emscripten::val>("concat", inputs, axis);
+  emscripten::val output = emscripten::val::undefined();
+  if (num_inputs <= 4 || model_builder.GetPreferredLayout() == DataLayout::NCHW) {
+    output = model_builder.GetBuilder().call<emscripten::val>("concat", emscripten::val::array(inputs), axis);
+  } else {
+    // WebNN XNNPack backend only supports the concat with inputs number <= 4,
+    // decomposing the Concat with inputs number > 4 into multiple WebNN concat ops.
+    size_t remaining_inputs = num_inputs;
+    size_t max_inputs = 4;
+    while (remaining_inputs > 0) {
+      std::vector<emscripten::val> chunk_inputs;
+
+      // Push the last concated output to the next chunk_inputs.
+      if (output != emscripten::val::undefined()) {
+        chunk_inputs.push_back(output);
+        max_inputs = 3;
+      }
+
+      size_t chunk_size = std::min(remaining_inputs, max_inputs);
+
+      for (size_t i = 0; i < chunk_size; i++) {
+        chunk_inputs.push_back(inputs[num_inputs - remaining_inputs + i]);
+      }
+
+      output = model_builder.GetBuilder().call<emscripten::val>("concat", emscripten::val::array(chunk_inputs), axis);
+      remaining_inputs -= chunk_size;
+    }
+  }
 
   model_builder.AddOperand(node.OutputDefs()[0]->Name(), std::move(output));
   return Status::OK();