Fix host/device calls for __half. The previous code had `__host__ __device__` functions calling `__device__` functions (e.g. `__low2half`) which caused build failures in tensorflow. Also tried to simplify the `#ifdef` guards to make them more clear.
Eigen is a C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.
For more information go to http://eigen.tuxfamily.org/.
For pull request, bug reports, and feature requests, go to https://gitlab.com/libeigen/eigen.