Summary:
This converts raysamplers to ReplaceableBase so that users can hack their own raysampling impls.
Context: Andrea tried to implement TensoRF within implicitron but could not due to the need to implement his own raysampler.
Reviewed By: shapovalov
Differential Revision: D36016318
fbshipit-source-id: ef746f3365282bdfa9c15f7b371090a5aae7f8da