Session* new_session;
  Status status = NewSession(session_options, &new_session);
  TF_CHECK_OK(status) << status;
  impl_.reset(new Impl(new_session, scope.graph_as_shared_ptr()));
  CHECK_NOTNULL(impl()->session_.get());
}

// Define destructor here so we can forward declare `Impl` in client_session.h.
// If we define a dtor in the header file or use the default dtor,
// unique_ptr<Impl> needs the complete type.