Fix DeformConvTester::test_backward_cuda by setting threads per block to 512 (#3942)

Author: NicolasHug

torchvision/csrc/ops/cuda/deform_conv2d_kernel.cu

@@ -85,10 +85,7 @@ inline unsigned int GET_THREADS() {
 #ifdef __HIP_PLATFORM_HCC__
   return 256;
 #endif
-  if (at::cuda::getCurrentDeviceProperties()->major >= 6) {
-    return 1024;
-  }
-  return 512;
+    return 512;
 }
 
 inline unsigned int GET_BLOCKS(