Submesh 0/n: Default to empty Meshes

Summary:
The default behavior of Meshes (with verts=None, faces=None) throws an exception:
```
meshes = Meshes()
> ValueError: Verts and Faces must be either a list or a tensor with shape (batch_size, N, 3) where N is either the maximum number of verts or faces respectively.
```

Instead, let's default to an empty mesh, following e.g. PyTorch:
```
empty_tensor = torch.FloatTensor()
> torch.tensor([])
```

this change is backwards-compatible (you can still init with verts=[], faces=[]).

Reviewed By: bottler, nikhilaravi

Differential Revision: D35443453

fbshipit-source-id: d638a8fef49a089bf0da6dd2201727b94ceb21ec

Author: Krzysztof Chalupka

pytorch3d/structures/meshes.py

@@ -214,8 +214,8 @@ class Meshes:
 
     def __init__(
         self,
-        verts=None,
-        faces=None,
+        verts,
+        faces,
         textures=None,
         *,
         verts_normals=None,

tests/test_meshes.py

@@ -265,9 +265,7 @@ def test_simple_random_meshes(self):
             self.assertTrue(mesh_to_edges_packed_first_idx[0] == 0)
 
     def test_allempty(self):
-        verts_list = []
-        faces_list = []
-        mesh = Meshes(verts=verts_list, faces=faces_list)
+        mesh = Meshes(verts=[], faces=[])
         self.assertEqual(len(mesh), 0)
         self.assertEqual(mesh.verts_padded().shape[0], 0)
         self.assertEqual(mesh.faces_padded().shape[0], 0)