Fixes bug when importing from ..common.types (#706)

Summary:
There are a couple times in the code where Device, make_device and get_device are imported by:
`from ..common.types import Device, get_device, make_device`
which will not work without this init file, at least on python 3.8.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/706

Reviewed By: bottler

Differential Revision: D29031835

Pulled By: patricklabatut

fbshipit-source-id: 15743e3c8cffdfcca51b6d2f377b923da9cbe6f9

pytorch3d/common/__init__.py

+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.