Move the CUDA implementation of log1p to ATen. (#26923)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26923
Fix #24588
Test Plan: Imported from OSS
Differential Revision: D17984184
Pulled By: VitalyFedyunin
fbshipit-source-id: 3bc2be4f08e800b1de274940f2bd3d5b418b45ee
