Fix compiler warning in knn.ku

Summary: This change updates the type of p2_idx from size_t to int64_t to address compiler warnings related to signed/unsigned comparison.

Reviewed By: bottler

Differential Revision: D52879393

fbshipit-source-id: de5484d78a907fccdaae3ce036b5e4a1a0a4de70

pytorch3d/csrc/knn/knn.cu

@@ -495,7 +495,7 @@ __global__ void KNearestNeighborBackwardKernel(
     if ((p1_idx < num1) && (k < num2)) {
       const float grad_dist = grad_dists[n * P1 * K + p1_idx * K + k];
       // index of point in p2 corresponding to the k-th nearest neighbor
-      const size_t p2_idx = idxs[n * P1 * K + p1_idx * K + k];
+      const int64_t p2_idx = idxs[n * P1 * K + p1_idx * K + k];
       // If the index is the pad value of -1 then ignore it
       if (p2_idx == -1) {
         continue;