TexturesAtlas in plotly

Summary:
Lets a K=1 textures atlas be viewed in plotly. Fixes #916 .

Test: Now get colored faces in
```
import torch
from pytorch3d.utils import ico_sphere
from pytorch3d.vis.plotly_vis import plot_batch_individually
from pytorch3d.renderer import TexturesAtlas

b = ico_sphere()
face_colors = torch.rand(b.faces_padded().shape)
tex = TexturesAtlas(face_colors[:,:,None,None])
b.textures=tex
plot_batch_individually(b)
```

Reviewed By: gkioxari

Differential Revision: D32190470

fbshipit-source-id: 258d30b7e9d79751a79db44684b5540657a2eff5

Author: bottler

pytorch3d/vis/plotly_vis.py

@@ -10,7 +10,12 @@
 import plotly.graph_objects as go
 import torch
 from plotly.subplots import make_subplots
-from pytorch3d.renderer import RayBundle, TexturesVertex, ray_bundle_to_ray_points
+from pytorch3d.renderer import (
+    RayBundle,
+    TexturesAtlas,
+    TexturesVertex,
+    ray_bundle_to_ray_points,
+)
 from pytorch3d.renderer.camera_utils import camera_to_eye_at_up
 from pytorch3d.renderer.cameras import CamerasBase
 from pytorch3d.structures import Meshes, Pointclouds, join_meshes_as_scene
@@ -580,13 +585,19 @@ def _add_mesh_trace(
     mesh = mesh.detach().cpu()
     verts = mesh.verts_packed()
     faces = mesh.faces_packed()
-    # If mesh has vertex colors defined as texture, use vertex colors
+    # If mesh has vertex colors or face colors, use them
     # for figure, otherwise use plotly's default colors.
     verts_rgb = None
+    faces_rgb = None
     if isinstance(mesh.textures, TexturesVertex):
         verts_rgb = mesh.textures.verts_features_packed()
         verts_rgb.clamp_(min=0.0, max=1.0)
         verts_rgb = torch.tensor(255.0) * verts_rgb
+    if isinstance(mesh.textures, TexturesAtlas):
+        atlas = mesh.textures.atlas_packed()
+        # If K==1
+        if atlas.shape[1] == 1 and atlas.shape[3] == 3:
+            faces_rgb = atlas[:, 0, 0]
 
     # Reposition the unused vertices to be "inside" the object
     # (i.e. they won't be visible in the plot).
@@ -602,6 +613,7 @@ def _add_mesh_trace(
             y=verts[:, 1],
             z=verts[:, 2],
             vertexcolor=verts_rgb,
+            facecolor=faces_rgb,
             i=faces[:, 0],
             j=faces[:, 1],
             k=faces[:, 2],