[Eager] fix lerp grad kernel logic (#44705)

paddle/phi/kernels/impl/lerp_grad_kernel_impl.h

@@ -34,14 +34,22 @@ static void LerpGradFunction(const Context& ctx,
   auto* dy = y_grad;
 
   auto dout_dims = dout.dims();
-  auto dx_dims = phi::funcs::ExtendDims2Rank(dx->dims(), D);
-  auto dy_dims = phi::funcs::ExtendDims2Rank(dy->dims(), D);
+  DDim dx_dims;
+  DDim dy_dims;
+
   auto w_dims = phi::funcs::ExtendDims2Rank(w.dims(), D);
   Eigen::DSizes<int, D> dx_bcast_dims;
   Eigen::DSizes<int, D> dy_bcast_dims;
   Eigen::DSizes<int, D> w_bcast_dims;
-  phi::funcs::GetBroadcastDims<D>(dx_dims, dout_dims, &dx_bcast_dims);
-  phi::funcs::GetBroadcastDims<D>(dy_dims, dout_dims, &dy_bcast_dims);
+
+  if (dx) {
+    dx_dims = phi::funcs::ExtendDims2Rank(dx->dims(), D);
+    phi::funcs::GetBroadcastDims<D>(dx_dims, dout_dims, &dx_bcast_dims);
+  }
+  if (dy) {
+    dy_dims = phi::funcs::ExtendDims2Rank(dy->dims(), D);
+    phi::funcs::GetBroadcastDims<D>(dy_dims, dout_dims, &dy_bcast_dims);
+  }
   phi::funcs::GetBroadcastDims<D>(w_dims, dout_dims, &w_bcast_dims);
 
   auto eigen_w = phi::EigenTensor<T, D>::From(w, w_dims);
@@ -50,11 +58,16 @@ static void LerpGradFunction(const Context& ctx,
   Eigen::DSizes<int, D * 2> dx_reshape_dims;
   Eigen::DSizes<int, D * 2> dy_reshape_dims;
   Eigen::DSizes<int, D> reduce_dims;
+
   for (int i = 0; i < dout_dims.size(); ++i) {
-    dx_reshape_dims[2 * i] = dx_bcast_dims[i];
-    dx_reshape_dims[2 * i + 1] = dx_dims[i];
-    dy_reshape_dims[2 * i] = dy_bcast_dims[i];
-    dy_reshape_dims[2 * i + 1] = dy_dims[i];
+    if (dx) {
+      dx_reshape_dims[2 * i] = dx_bcast_dims[i];
+      dx_reshape_dims[2 * i + 1] = dx_dims[i];
+    }
+    if (dy) {
+      dy_reshape_dims[2 * i] = dy_bcast_dims[i];
+      dy_reshape_dims[2 * i + 1] = dy_dims[i];
+    }
     reduce_dims[i] = 2 * i;
   }