Add gather_point backward npu adpater

Add gather_point backward npu adpater.

mmcv/ops/csrc/pytorch/npu/gather_points_npu.cpp

@@ -22,8 +22,48 @@ void gather_points_forward_npu(int b, int c, int n, int npoints,
       .Run();
 }
 
+void gather_points_backward_npu(int b, int c, int n, int npoints,
+                                 const Tensor grad_out, const Tensor idx,
+                                 Tensor grad_points) {
+  at::Tensor indices = idx;
+  if (idx.scalar_type() != at::ScalarType::Int) {
+    indices = idx.to(at::kInt);
+  }
+  if (idx.dim() == 0) {
+    indices.unsqueeze_(0);
+  }
+  int64_t dim = 0;
+  at::SmallVector<int64_t, N> pad_size = array_to_small_vector(idx.sizes());
+  at::Tensor trans_grad_points = grad_points.transpose(1,2).contiguous();
+  at::Tensor grad_points_view = trans_grad_points.view({trans_grad_points.sizes()[0]*trans_grad_points.sizes()[1], trans_grad_points.sizes()[2]});
+  at::Tensor trans_grad_out = grad_out.transpose(1,2).contiguous();
+  trans_grad_out = trans_grad_out.view({trans_grad_out.sizes()[0]*trans_grad_out.sizes()[1], trans_grad_out.sizes()[2]});
+  auto index = at::arange(0,b);
+  index = index.to(grad_out.device());
+  index = at::mul(index,n);
+  index = index.view({b,1});
+  index = at::broadcast_to(index, pad_size);
+  indices = at::add(index, indices);
+  indices = indices.view({-1});
+  OpCommand cmd;
+  cmd.Name("InplaceIndexAdd")
+      .Input(grad_points_view)
+      .Input(indices)
+      .Input(trans_grad_out)
+      .Output(grad_points_view)
+      .Attr("axis", dim)
+      .Run();
+  at::Tensor grad_points_result = grad_points_view.view(trans_grad_points.sizes());
+  grad_points_result = grad_points_result.transpose(1,2);
+  grad_points.copy_(grad_points_result);
+}
+
 void gather_points_forward_impl(int b, int c, int n, int npoints,
                                 const Tensor points, const Tensor idx,
                                 Tensor out);
+void gather_points_backward_impl(int b, int c, int n, int npoints,
+                                 const Tensor grad_out, const Tensor idx,
+                                 Tensor grad_points);
 
 REGISTER_NPU_IMPL(gather_points_forward_impl, gather_points_forward_npu);
+REGISTER_NPU_IMPL(gather_points_backward_impl, gather_points_backward_npu);