different learning rate for different parts

Summary:
Adds the ability to have different learning rates for different parts of the model. The trainable parts of the implicitron have a new member

       param_groups: dictionary where keys are names of individual parameters,
            or module’s members and values are the parameter group where the
            parameter/member will be sorted to. "self" key is used to denote the
            parameter group at the module level. Possible keys, including the "self" key
            do not have to be defined. By default all parameters are put into "default"
            parameter group and have the learning rate defined in the optimizer,
            it can be overriden at the:
                - module level with “self” key, all the parameters and child
                    module s parameters will be put to that parameter group
                - member level, which is the same as if the `param_groups` in that
                    member has key=“self” and value equal to that parameter group.
                    This is useful if members do not have `param_groups`, for
                    example torch.nn.Linear.
                - parameter level, parameter with the same name as the key
                    will be put to that parameter group.

And in the optimizer factory, parameters and their learning rates are recursively gathered.

Reviewed By: shapovalov

Differential Revision: D40145802

fbshipit-source-id: 631c02b8d79ee1c0eb4c31e6e42dbd3d2882078a

tests/implicitron/test_voxel_grids.py

@@ -19,7 +19,6 @@ from pytorch3d.implicitron.models.implicit_function.utils import (
 from pytorch3d.implicitron.models.implicit_function.voxel_grid import (
     CPFactorizedVoxelGrid,
     FullResolutionVoxelGrid,
-    FullResolutionVoxelGridValues,
     VMFactorizedVoxelGrid,
     VoxelGridModule,
 )