Removes THCNumerics usages in RNN.cu (#15085)
Summary:
We don't need THCNumerics here since at::Half can be implicitly converted to float and the cuda math dispatches are handled by `/usr/local/cuda/include/crt/math_functions.hpp` and `cmath`. ATen should be free of THCNumerics after this and when porting kernels from THC, one should not use THCNumerics.
Should close: https://github.com/pytorch/pytorch/issues/11878
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15085
Differential Revision: D13447558
Pulled By: soumith
fbshipit-source-id: 4ff5cbf838edcd01e2d1397e4d7f4f920e9e9fc3
