GitLab

Summary:
When a list of Meshes is `join_batched()`, the `num_verts_per_mesh` in the list would be unexpectedly modified.

Also some cleanup around `_num_verts_per_mesh`.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/623

Test Plan: A modification to an existing test checks this.

Reviewed By: nikhilaravi

Differential Revision: D27682104

Pulled By: bottler

fbshipit-source-id: 9d00913dfb4869bd6c7d3f5cc9156b7b6f1aecc9