updates based on PR feedback

There are two major changes (and a few minor ones which are not listed here...see PR discussion for details)

1. Eigen::half implementations for HIP and CUDA have been merged.
This means that
- `CUDA/Half.h` and `HIP/hcc/Half.h` got merged to a new file `GPU/Half.h`
- `CUDA/PacketMathHalf.h` and `HIP/hcc/PacketMathHalf.h` got merged to a new file `GPU/PacketMathHalf.h`
- `CUDA/TypeCasting.h` and `HIP/hcc/TypeCasting.h` got merged to a new file `GPU/TypeCasting.h`

After this change the `HIP/hcc` directory only contains one file `math_constants.h`. That will go away too once that file becomes a part of the HIP install.

2. new macros EIGEN_GPUCC, EIGEN_GPU_COMPILE_PHASE and EIGEN_HAS_GPU_FP16 have been added and the code has been updated to use them where appropriate.
- `EIGEN_GPUCC` is the same as `(EIGEN_CUDACC || EIGEN_HIPCC)`
- `EIGEN_GPU_DEVICE_COMPILE` is the same as `(EIGEN_CUDA_ARCH || EIGEN_HIP_DEVICE_COMPILE)`
- `EIGEN_HAS_GPU_FP16` is the same as `(EIGEN_HAS_CUDA_FP16 or EIGEN_HAS_HIP_FP16)`

1 files changed, 2 insertions, 2 deletions

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorScan.h b/unsupported/Eigen/CXX11/src/Tensor/TensorScan.h
index 174a6a064..6d68e256f 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorScan.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorScan.h
@@ -242,7 +242,7 @@ struct ScanLauncher {
   }
 };
 
-#if defined(EIGEN_USE_GPU) && (defined(EIGEN_CUDACC) || defined(EIGEN_HIPCC))
+#if defined(EIGEN_USE_GPU) && (defined(EIGEN_GPUCC))
 
 // GPU implementation of scan
 // TODO(ibab) This placeholder implementation performs multiple scans in
@@ -286,7 +286,7 @@ struct ScanLauncher<Self, Reducer, GpuDevice> {
 #endif     
   }
 };
-#endif  // EIGEN_USE_GPU && (EIGEN_CUDACC || EIGEN_HIPCC)
+#endif  // EIGEN_USE_GPU && (EIGEN_GPUCC)
 
 }  // end namespace Eigen