Summary: As reported in https://github.com/facebookresearch/pytorch3d/pull/1100, a rasterizer couldn't be moved if it was missing the optional cameras member. Fix that. This matters because the renderer.to calls rasterizer.to, so this to() could be called even by a user who never sets a cameras member.

Reviewed By: nikhilaravi

Differential Revision: D34643841

fbshipit-source-id: 7e26e32e8bc585eb1ee533052754a7b59bc7467a