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Summary:
This fixes a indexing bug in HardDepthShader and adds proper unit tests for both of the DepthShaders. This bug was introduced when updating the shader sizes and discovered when I switched my local model onto pytorch3d trunk instead of the patched copy.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1252

Test Plan:
Unit test + custom model code

```
pytest tests/test_shader.py
```

![image](https://user-images.githubusercontent.com/909104/178397456-f478d0e0-9f6c-467a-a85b-adb4c47adfee.png)

Reviewed By: bottler

Differential Revision: D37775767

Pulled By: d4l3k

fbshipit-source-id: 5f001903985976d7067d1fa0a3102d602790e3e8