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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

Co-authored-by: Jeremy Francis Reizenstein <bottler@users.noreply.github.com>