remove legacy cross() usage (#20)

Summary:
Pull Request resolved: #20

Fix legacy cross usage. Future cross() call must have explicit dim parameter.

Currently it has warning:
```
Warning: Using torch.cross without specifying the dim arg is deprecated.
Please either pass the dim explicitly or simply use torch.linalg.cross.
The default value of dim will change to agree with that of linalg.cross in a future release. (function operator())
```

Reviewed By: yutingye

Differential Revision: D58848086

fbshipit-source-id: 9976650c62ab75bd894a853ad81e3595fa2e5c57

pymomentum/tensor_momentum/tensor_skinning.cpp

@@ -401,7 +401,7 @@ at::Tensor computeVertexNormals(
   at::Tensor x3 =
       at::index_select(vertex_positions, -2, triangles.select(-1, 2));
 
-  at::Tensor triangle_normals = at::cross(x2 - x1, x3 - x1);
+  at::Tensor triangle_normals = at::cross(x2 - x1, x3 - x1, -1);
   at::Tensor vertex_normals = at::zeros_like(vertex_positions);
   for (int64_t i = 0; i < 3; ++i) {
     vertex_normals.index_add_(-2, triangles.select(-1, i), triangle_normals);