MC rasterize supports heterogeneous bundle; refactoring of bundle-to-padded

Summary:
Rasterize MC was not adapted to heterogeneous bundles.

There are some caveats though:
1) on CO3D, we get up to 18 points per image, which is too few for a reasonable visualisation (see below);
2) rasterising for a batch of 100 is slow.

I also moved the unpacking code close to the bundle to be able to reuse it.

{F789678778}

Reviewed By: bottler, davnov134

Differential Revision: D41008600

fbshipit-source-id: 9f10f1f9f9a174cf8c534b9b9859587d69832b71

tests/test_packed_to_padded.py

@@ -188,6 +188,16 @@ class TestPackedToPadded(TestCaseMixin, unittest.TestCase):
         # check forward
         self.assertClose(values_packed, values_packed_torch)
 
+        if len(dims) > 0:
+            values_packed_dim2 = padded_to_packed(
+                values.transpose(1, 2),
+                mesh_to_faces_packed_first_idx,
+                num_faces_per_mesh.sum().item(),
+                max_size_dim=2,
+            )
+            # check forward
+            self.assertClose(values_packed_dim2, values_packed_torch)
+
         # check backward
         if len(dims) == 0:
             grad_inputs = torch.rand((num_faces_per_mesh.sum().item()), device=device)