Annotate dunder functions

Summary: Annotate the (return type of the) following dunder functions across the codebase: `__init__()`, `__len__()`, `__getitem__()`

Reviewed By: nikhilaravi

Differential Revision: D29001801

fbshipit-source-id: 928d9e1c417ffe01ab8c0445311287786e997c7c

pytorch3d/ops/graph_conv.py

@@ -21,7 +21,7 @@ class GraphConv(nn.Module):
         output_dim: int,
         init: str = "normal",
         directed: bool = False,
-    ):
+    ) -> None:
         """
         Args:
             input_dim: Number of input features per vertex.

pytorch3d/ops/marching_cubes.py

@@ -15,7 +15,7 @@ EPS = 0.00001
 
 
 class Cube:
-    def __init__(self, bfl_vertex: Tuple[int, int, int], spacing: int = 1):
+    def __init__(self, bfl_vertex: Tuple[int, int, int], spacing: int = 1) -> None:
         """
         Initializes a cube given the bottom front left vertex coordinate
         and the cube spacing

pytorch3d/ops/subdivide_meshes.py

@@ -26,7 +26,7 @@ class SubdivideMeshes(nn.Module):
     but different vertex positions.
     """
 
-    def __init__(self, meshes=None):
+    def __init__(self, meshes=None) -> None:
         """
         Args:
             meshes: Meshes object or None. If a meshes object is provided,