GitLab

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