fix atan2 grad (#56067)

paddle/phi/kernels/impl/atan2_grad_kernel_impl.h

@@ -32,9 +32,13 @@ struct Atan2GradFunctor {
     float x1 = static_cast<float>(x1_[idx]);
     float x2 = static_cast<float>(x2_[idx]);
     float x = x1 * x1 + x2 * x2;
+    if (dx1_) {
       dx1_[idx] = static_cast<T>(static_cast<float>(dout_[idx]) * x2 / x);
+    }
+    if (dx2_) {
       dx2_[idx] = static_cast<T>(-static_cast<float>(dout_[idx]) * x1 / x);
     }
+  }
 
   const T* x1_;
   const T* x2_;
@@ -56,9 +60,13 @@ struct Atan2GradFunctor<double> {
 
   HOSTDEVICE void operator()(int64_t idx) const {
     auto x = x1_[idx] * x1_[idx] + x2_[idx] * x2_[idx];
+    if (dx1_) {
       dx1_[idx] = dout_[idx] * x2_[idx] / x;
+    }
+    if (dx2_) {
       dx2_[idx] = -dout_[idx] * x1_[idx] / x;
     }
+  }
 
   const double* x1_;
   const double* x2_;
@@ -81,9 +89,11 @@ void Atan2GradKernel(const Context& ctx,
   auto out_grad_data = out_grad.data<T>();
 
   auto* x_grad_data =
-      ctx.template Alloc<T>(x_grad, size_t(x.numel() * sizeof(T)));
+      x_grad ? ctx.template Alloc<T>(x_grad, size_t(x.numel() * sizeof(T)))
+             : nullptr;
   auto* y_grad_data =
-      ctx.template Alloc<T>(y_grad, size_t(y.numel() * sizeof(T)));
+      y_grad ? ctx.template Alloc<T>(y_grad, size_t(y.numel() * sizeof(T)))
+             : nullptr;
 
   phi::funcs::ForRange<Context> for_range(ctx, numel);
   phi::Atan2GradFunctor<T> functor(