chore: import upstream snapshot with attribution

paddle/phi/kernels/impl/softmax_grad_kernel_impl.h

@@ -0,0 +1,57 @@
+/* Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. */
+
+#pragma once
+
+#include "paddle/phi/kernels/funcs/axis_utils.h"
+#include "paddle/phi/kernels/funcs/math_function.h"
+#include "paddle/phi/kernels/funcs/softmax.h"
+#include "paddle/phi/kernels/softmax_grad_kernel.h"
+
+namespace phi {
+
+template <typename T, typename Context>
+void SoftmaxGradKernel(const Context& dev_ctx,
+                       const DenseTensor& out,
+                       const DenseTensor& out_grad,
+                       int axis,
+                       DenseTensor* x_grad) {
+  dev_ctx.template Alloc<T>(x_grad);
+
+  const int rank = x_grad->dims().size();
+  // For 0D Tensor
+  if (rank == 0) {
+    funcs::set_constant(dev_ctx, x_grad, static_cast<T>(0.0));
+    return;
+  }
+  // For zero-sized Tensor
+  if (x_grad->numel() == 0) {
+    return;
+  }
+
+  const int calc_axis = funcs::CanonicalAxis(axis, rank);
+  int64_t axis_dim = x_grad->dims()[calc_axis];
+
+  const int64_t n = funcs::SizeToAxis(calc_axis, x_grad->dims());
+  const int64_t d = funcs::SizeFromAxis(calc_axis, x_grad->dims());
+  DenseTensor dX_2d, Out_2d, dOut_2d;
+  dX_2d.ShareDataWith(*x_grad).Resize({n, d});
+  Out_2d.ShareDataWith(out).Resize({n, d});
+  dOut_2d.ShareDataWith(out_grad).Resize({n, d});
+
+  funcs::SoftmaxGradFunctor<Context, T>()(
+      dev_ctx, axis_dim, &Out_2d, &dOut_2d, &dX_2d);
+}
+
+}  // namespace phi