Updates corresponding to the latest round of PR feedback

The major changes are

1. Moving CUDA/PacketMath.h to GPU/PacketMath.h
2. Moving CUDA/MathFunctions.h to GPU/MathFunction.h
3. Moving CUDA/CudaSpecialFunctions.h to GPU/GpuSpecialFunctions.h
    The above three changes effectively enable the Eigen "Packet" layer for the HIP platform

4. Merging the "hip_basic" and "cuda_basic" unit tests into one ("gpu_basic")
5. Updating the "EIGEN_DEVICE_FUNC" marking in some places

The change has been tested on the HIP and CUDA platforms.

Eigen/src/Core/MathFunctions.h

@@ -982,7 +982,12 @@ template<>
 EIGEN_DEVICE_FUNC
 EIGEN_ALWAYS_INLINE long double mini(const long double& x, const long double& y)
 {
+#if defined(EIGEN_HIPCC)
+  // no "fminl" on HIP yet
+  return (x < y) ? x : y;
+#else
   return fminl(x, y);
+#endif
 }
 
 template<typename T>
@@ -1007,7 +1012,12 @@ template<>
 EIGEN_DEVICE_FUNC
 EIGEN_ALWAYS_INLINE long double maxi(const long double& x, const long double& y)
 {
+#if defined(EIGEN_HIPCC)
+  // no "fmaxl" on HIP yet
+  return (x > y) ? x : y;
+#else
   return fmaxl(x, y);
+#endif
 }
 #endif