This change fixes random segfaults in graph tests that
are seen after the change make internal callbacks non-blocking.
The callback thread that decreases the GraphExec ref count
may now run after the runtime shutdown. This can cause a segfault
because the hip::device that is accessed in GraphExec destructor
is already destroyed during runtime shutdown. This patch ensures
that the hip::device object stays alive until after the
callback thread completes.
Change-Id: I75a6ac01f27a0b2250bbd10ed389ebfb322927af
Ioannis Assiouras
21c223f8df