Support other face / vertex attributes in save_{obj,ply} function

## 🚀 Feature

Support other texture representations (at least the `TexturesVertex` one) in `save_obj` function

https://github.com/facebookresearch/pytorch3d/blob/1872e0249c8c2c4eef7088996284a16cab52ca25/pytorch3d/io/obj_io.py#L652

## Motivation

Saving a mesh without loosing the texture information, e.g. as done in `fit_textured_mesh.ipynb`

## Pitch

For the TexturesVertex case it should be simple, the beginning of the `_save` function can for example be replaced by:

```
def _save(
f,
verts,
faces,
decimal_places: Optional[int] = None,
*,
verts_rgb: Optional[torch.Tensor] = None,
verts_uvs: Optional[torch.Tensor] = None,
faces_uvs: Optional[torch.Tensor] = None,
save_texture: bool = False,
) -> None:

if len(faces) and (faces.dim() != 2 or faces.size(1) != 3):
message = "'faces' should either be empty or of shape (num_faces, 3)."
raise ValueError(message)

if not (len(verts) or len(faces)):
warnings.warn("Empty 'verts' and 'faces' arguments provided")
return

if verts_rgb is not None and (verts_rgb.dim() != 2 or verts_rgb.size(1) != 3):
message = "'verts_rgb' should either be empty or of shape (num_verts, 3)."
raise ValueError(message)

verts, faces = verts.cpu(), faces.cpu()

lines = ""

if len(verts):
if decimal_places is None:
float_str = "%f"
else:
float_str = "%" + ".%df" % decimal_places

V, D = verts.shape
for i in range(V):
vert = [float_str % verts[i, j] for j in range(D)]
if verts_rgb is not None:
vert += [float_str % verts_rgb[i, j] for j in range(3)]
lines += "v %s\n" % " ".join(vert)
```

modulo some verifications that `verts_rgb` has values in [0, 1]