Add rescaling to voxel grids

Summary: Any module can be subscribed to step updates from the training loop. Once the training loop publishes a step the voxel grid changes its dimensions. During the construction of VoxelGridModule and its parameters it does not know which is the resolution that will be loaded from checkpoint, so before the checkpoint loading a hook runs which changes the VoxelGridModule's parameters to match shapes of the loaded checkpoint.

Reviewed By: bottler

Differential Revision: D39026775

fbshipit-source-id: 0d359ea5c8d2eda11d773d79c7513c83585d5f17

Author: Darijan Gudelj