Flexible #channels in TexturesUV.join_scene

Summary: The TexturesUV class supports an arbitrary number of channels. In one place in join_scene we unnecessarily assumed the usual value, 3.

Reviewed By: patricklabatut

Differential Revision: D29133477

fbshipit-source-id: de8eb15fdd55675da084634d9d99e2a3f4d35401

Author: bottler

pytorch3d/renderer/mesh/textures.py

@@ -1121,8 +1121,9 @@ def join_scene(self) -> "TexturesUV":
                 )
             )
         merging_plan = pack_unique_rectangles(heights_and_widths)
+        C = maps[0].shape[-1]
         # pyre-fixme[16]: `Tensor` has no attribute `new_zeros`.
-        single_map = maps[0].new_zeros((*merging_plan.total_size, 3))
+        single_map = maps[0].new_zeros((*merging_plan.total_size, C))
         verts_uvs = self.verts_uvs_list()
         verts_uvs_merged = []