Allow DIY to compile under ROCm HIP compiler

The ROCm HIP compiler ( a clang based compiler ) has issues compiling DIY via VTK-m. In general the issue is a compiler bug where it incorrectly presumes that the diy/master constructor is being invoked from a device side function.

This is a problem as HIP doesn't support allocations or frees ( so no keyword new / delete ) in device side functions. We work around the compiler bug by making the allocation happen in the definition instead of the declaration.