Move the CUDA implementation of log2 to ATen. (#26769)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26769
Fix #24589
Test Plan: Imported from OSS
Differential Revision: D17960122
Pulled By: VitalyFedyunin
fbshipit-source-id: 58dff236886bbf3a0a152d7422aa8a5c478ee1de
