Fixing a bug that prevents opacity gradient calculation if no other gradients are required.

Summary: An early-return test for gradient calculation did not include the opacity gradient calculation - hence would also return early without calculating gradients even if opacity gradients are required.

Reviewed By: bottler

Differential Revision: D29505684

fbshipit-source-id: 575e820b8f58b19476b2fe3288702806733e840b

Author: classner

pytorch3d/csrc/pulsar/pytorch/renderer.cpp

@@ -1046,7 +1046,7 @@ Renderer::backward(
       at::optional<torch::Tensor>,
       at::optional<torch::Tensor>>
       ret;
-  if (mode == 1 || (!dif_pos && !dif_col && !dif_rad && !dif_cam)) {
+  if (mode == 1 || (!dif_pos && !dif_col && !dif_rad && !dif_cam && !dif_opy)) {
     return ret;
   }
   // Create the camera information.