Move THCTensor_(exponential) to ATen (#21297)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21297
ghimport-source-id: 5f45154e714ab44dec961dabf1c64e54aaa063a2
Reviewed By: jerryzh168
Differential Revision: D15632931
Pulled By: ezyang
fbshipit-source-id: 0367eec0a9ef6812b1b3ab7597817ee40a011bb8
