Shader API more consistent naming

Summary:
Renamed shaders to be prefixed with Hard/Soft depending on if they use a probabalistic blending (Soft) or use the closest face (Hard).

There is some code duplication but I thought it would be cleaner to have separate shaders for each task rather than:
- inheritance (which we discussed previously that we want to avoid)
- boolean (hard/soft) or a string (hard/soft) - new blending functions other than the ones provided would need if statements in the current shaders which might get messy.

Also added a `flat_shading` function and a `FlatShader` - I could make this into a tutorial as it was really easy to add a new shader and it might be a nice showcase.

NOTE: There are a few more places where the naming will need to change (e.g the tutorials) but I wanted to reach a consensus on this before changing it everywhere.

Reviewed By: jcjohnson

Differential Revision: D19761036

fbshipit-source-id: f972f6530c7f66dc5550b0284c191abc4a7f6fc4

tests/test_rendering_meshes.py

@@ -25,10 +25,10 @@ from pytorch3d.renderer.mesh.rasterizer import (
 from pytorch3d.renderer.mesh.renderer import MeshRenderer
 from pytorch3d.renderer.mesh.shader import (
     BlendParams,
-    GouradShader,
-    PhongShader,
-    SilhouetteShader,
-    TexturedPhongShader,
+    HardGouradShader,
+    HardPhongShader,
+    SoftSilhouetteShader,
+    TexturedSoftPhongShader,
 )
 from pytorch3d.renderer.mesh.texturing import Textures
 from pytorch3d.structures.meshes import Meshes
@@ -92,7 +92,7 @@ class TestRenderingMeshes(unittest.TestCase):
         )
         renderer = MeshRenderer(
             rasterizer=rasterizer,
-            shader=PhongShader(
+            shader=HardPhongShader(
                 lights=lights, cameras=cameras, materials=materials
             ),
         )
@@ -133,7 +133,7 @@ class TestRenderingMeshes(unittest.TestCase):
         lights.location = torch.tensor([0.0, 0.0, -2.0], device=device)[None]
         renderer = MeshRenderer(
             rasterizer=rasterizer,
-            shader=GouradShader(
+            shader=HardGouradShader(
                 lights=lights, cameras=cameras, materials=materials
             ),
         )
@@ -197,7 +197,7 @@ class TestRenderingMeshes(unittest.TestCase):
             rasterizer=MeshRasterizer(
                 cameras=cameras, raster_settings=raster_settings
             ),
-            shader=PhongShader(
+            shader=HardPhongShader(
                 lights=lights, cameras=cameras, materials=materials
             ),
         )
@@ -242,7 +242,7 @@ class TestRenderingMeshes(unittest.TestCase):
             rasterizer=MeshRasterizer(
                 cameras=cameras, raster_settings=raster_settings
             ),
-            shader=SilhouetteShader(blend_params=blend_params),
+            shader=SoftSilhouetteShader(blend_params=blend_params),
         )
         images = renderer(sphere_mesh)
         alpha = images[0, ..., 3].squeeze().cpu()
@@ -296,7 +296,7 @@ class TestRenderingMeshes(unittest.TestCase):
             rasterizer=MeshRasterizer(
                 cameras=cameras, raster_settings=raster_settings
             ),
-            shader=TexturedPhongShader(
+            shader=TexturedSoftPhongShader(
                 lights=lights, cameras=cameras, materials=materials
             ),
         )