Add `buildRescale` helper function with scale width and double rounding option (#567)

Summary:
These scaling options can be used by lowering of convolution.

Pull Request resolved: https://github.com/pytorch/executorch/pull/567

Reviewed By: larryliu0820

Differential Revision: D49897715

Pulled By: digantdesai

fbshipit-source-id: f833aab567bdc4d227231f5845c4f0ff42bf2be1