fix pooling max grad

mindspore/ccsrc/backend/kernel_compiler/cpu/mkldnn/pooling_max_grad_cpu_kernel.cc

@@ -117,15 +117,12 @@ bool MaxPoolingGradCPUKernel::Launch(const std::vector<kernel::AddressPtr> &inpu
   size_t src_wh = src_shape_[2] * src_shape_[3];
   size_t dst_wh = dst_shape_[2] * dst_shape_[3];
   for (size_t n = 0; n < src_shape_[0]; ++n) {
-    auto task = [&](size_t start, size_t end) {
-      for (size_t c = start; c < end; ++c) {
-        ChannelPoolingGrad(input, diff, output);
-        input = input + src_wh;
-        output = output + src_wh;
-        diff = diff + dst_wh;
-      }
-    };
-    CPUKernelUtils::ParallelFor(task, src_shape_[1]);
+    for (size_t c = 0; c < src_shape_[1]; ++c) {
+      ChannelPoolingGrad(input, diff, output);
+      input = input + src_wh;
+      output = output + src_wh;
+      diff = diff + dst_wh;
+    }
   }
   return true;
 }