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Summary:
When the ply format looks as follows:
  ```
comment TextureFile ***.png
element vertex 892
property double x
property double y
property double z
property double nx
property double ny
property double nz
property double texture_u
property double texture_v
```
`MeshPlyFormat` class will read uv from the ply file and read the uv map as commented as TextureFile.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1100

Reviewed By: MichaelRamamonjisoa

Differential Revision: D50885176

Pulled By: bottler

fbshipit-source-id: be75b1ec9a17a1ed87dbcf846a9072ea967aec37