[CPU]Improve dynamic 4bit moe performance (#27240)

Signed-off-by: Zhang Xiangze <Xiangze.Zhang@arm.com>

csrc/moe/dynamic_4bit_int_moe_cpu.cpp

@@ -87,30 +87,23 @@ torch::Tensor dynamic_4bit_int_moe_cpu(
   const int64_t g_eff_13 = (group_size != -1) ? group_size : H;
   const int64_t g_eff_2 = (group_size != -1) ? group_size : I;
 
-  // Per-expert outputs filled in parallel
-  std::vector<torch::Tensor> y_list(E);
-  y_list.resize(E);
+  auto X_all = x_c.index_select(/*dim=*/0, expert_tokens);
+  if (apply_router_weight_on_input) {
+    X_all = X_all.mul(expert_gates.unsqueeze(1));
+  }
+  auto Y_all = at::empty({offsets[E], H}, x_c.options());
 
   at::parallel_for(0, E, 1, [&](int64_t e_begin, int64_t e_end) {
+    c10::InferenceMode guard;
     for (int64_t e = e_begin; e < e_end; ++e) {
       const int64_t te = counts[e];
       if (te == 0) {
-        y_list[e] = at::empty({0, H}, x_c.options());
         continue;
       }
 
       const int64_t start = offsets[e];
 
-      auto sel_tokens =
-          expert_tokens.narrow(/*dim=*/0, /*start=*/start, /*length=*/te);
-      auto gates_e =
-          expert_gates.narrow(/*dim=*/0, /*start=*/start, /*length=*/te);
-
-      auto x_e = x_c.index_select(/*dim=*/0, sel_tokens);
-
-      if (apply_router_weight_on_input) {
-        x_e = x_e.mul(gates_e.unsqueeze(1));
-      }
+      auto x_e = X_all.narrow(/*dim=*/0, /*start=*/start, /*length=*/te);
 
       auto w13_e = w13_packed.select(/*dim=*/0, e);
       auto w2_e = w2_packed.select(/*dim=*/0, e);
@@ -137,17 +130,15 @@ torch::Tensor dynamic_4bit_int_moe_cpu(
       // W2
       auto y = mm(act, w2_e, g_eff_2, /*in_features=*/I, /*out_features=*/H);
 
-      if (!apply_router_weight_on_input) {
-        y = y.mul(gates_e.unsqueeze(1));
-      }
-
       // Store per-expert result
-      y_list[e] = y;
+      Y_all.narrow(/*dim=*/0, /*start=*/start, /*length=*/te).copy_(y);
     }
   });
 
-  // Concatenate all expert outputs to match expert_tokens order
-  auto Y_all = at::cat(y_list, /*dim=*/0);
+  if (!apply_router_weight_on_input) {
+    Y_all = Y_all.mul(expert_gates.unsqueeze(1));
+  }
+
   auto out = at::zeros({T, H}, x.options());
   out =
       at::index_add(out, /*dim=*/0, /*index=*/expert_tokens, /*source=*/Y_all);