shader: fix HardDepthShader sizes + tests (#1252)

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

pytorch3d/renderer/mesh/shader.py

@@ -374,11 +374,11 @@ class HardDepthShader(ShaderBase):
         cameras = super()._get_cameras(**kwargs)
 
         zfar = kwargs.get("zfar", getattr(cameras, "zfar", 100.0))
-        mask = fragments.pix_to_face < 0
+        mask = fragments.pix_to_face[..., 0:1] < 0
 
-        zbuf = fragments.zbuf[..., 0].clone()
+        zbuf = fragments.zbuf[..., 0:1].clone()
         zbuf[mask] = zfar
-        return zbuf.unsqueeze(3)
+        return zbuf
 
 
 class SoftDepthShader(ShaderBase):

tests/test_shader.py

@@ -91,3 +91,35 @@ class TestShader(TestCaseMixin, unittest.TestCase):
 
             with self.assertRaises(ValueError):
                 shader(fragments, meshes)
+
+    def test_depth_shader(self):
+        shader_classes = [
+            HardDepthShader,
+            SoftDepthShader,
+        ]
+
+        verts = torch.tensor(
+            [[-1, -1, 0], [1, -1, 1], [1, 1, 0], [-1, 1, 1]], dtype=torch.float32
+        )
+        faces = torch.tensor([[0, 1, 2], [2, 3, 0]], dtype=torch.int64)
+        meshes = Meshes(verts=[verts], faces=[faces])
+
+        pix_to_face = torch.tensor([0, 1], dtype=torch.int64).view(1, 1, 1, 2)
+        barycentric_coords = torch.tensor(
+            [[0.1, 0.2, 0.7], [0.3, 0.5, 0.2]], dtype=torch.float32
+        ).view(1, 1, 1, 2, -1)
+        for faces_per_pixel in [1, 2]:
+            fragments = Fragments(
+                pix_to_face=pix_to_face[:, :, :, :faces_per_pixel],
+                bary_coords=barycentric_coords[:, :, :, :faces_per_pixel],
+                zbuf=torch.ones_like(pix_to_face),
+                dists=torch.ones_like(pix_to_face),
+            )
+            R, T = look_at_view_transform()
+            cameras = PerspectiveCameras(R=R, T=T)
+
+            for shader_class in shader_classes:
+                shader = shader_class()
+
+                out = shader(fragments, meshes, cameras=cameras)
+                self.assertEqual(out.shape, (1, 1, 1, 1))