ROCm/HIP specfic fixes + updates

1. Eigen/src/Core/arch/GPU/Half.h

   Updating the HIPCC implementation half so that it can declared as a __shared__ variable


2. Eigen/src/Core/util/Macros.h, Eigen/src/Core/util/Memory.h

   introducing a EIGEN_USE_STD(func) macro that calls
   - std::func be default
   - ::func when eigen is being compiled with HIPCC

   This change was requested in the previous HIP PR
   (https://bitbucket.org/eigen/eigen/pull-requests/518/pr-with-hip-specific-fixes-for-the-eigen/diff)


3. unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h

   Removing EIGEN_DEVICE_FUNC attribute from pure virtual methods as it is not supported by HIPCC


4. unsupported/Eigen/CXX11/src/Tensor/TensorReduction.h

   Disabling the template specializations of InnerMostDimReducer as they run into HIPCC link errors

1 files changed, 6 insertions, 2 deletions

diff --git a/Eigen/src/Core/arch/GPU/Half.h b/Eigen/src/Core/arch/GPU/Half.h
index f87d8a18c..7873f8ec0 100644
--- a/Eigen/src/Core/arch/GPU/Half.h
+++ b/Eigen/src/Core/arch/GPU/Half.h
@@ -52,7 +52,9 @@ namespace half_impl {
 #if !defined(EIGEN_HAS_GPU_FP16)
 // Make our own __half_raw definition that is similar to CUDA's.
 struct __half_raw {
-  EIGEN_DEVICE_FUNC __half_raw() : x(0) {}
+  // The default constructor cannot initialize its member, otherwise the
+  // derived class Eigen::Half cannot be used as __shared__ variable in HIPCC.
+  EIGEN_DEVICE_FUNC __half_raw() {}
   explicit EIGEN_DEVICE_FUNC __half_raw(unsigned short raw) : x(raw) {}
   unsigned short x;
 };
@@ -70,7 +72,9 @@ struct __half_raw {
 // so we need to implicitly convert "__half_raw" to "__half" to avoid having to explicitly make 
 // that conversiion in each call to a "__h*" routine...that is why we have "operator __half" routine
 struct __half_raw {
-  EIGEN_DEVICE_FUNC __half_raw() : x(0) {}
+  // The default constructor cannot initialize its member, otherwise the
+  // derived class Eigen::Half cannot be used as __shared__ variable in HIPCC.
+  EIGEN_DEVICE_FUNC __half_raw() {}
   explicit EIGEN_DEVICE_FUNC __half_raw(unsigned short raw) : x(raw) {}
   union {
     unsigned short x;