Using buffers inside a command graph potentially causes a copy back to host to be triggered when the buffer is destroyed or synchronized in some other way. This presents issues for the current graphs implementation, which causes an error to be thrown similar to #24. The cause is slightly different in this case though:

• The copy back operation tries to execute, but is instead added to the command_graph's command list
• The runtime then tries to immediately wait on the copy back operation which fails because nothing was enqueued and no host-visible event created.

This is the same problem preventing any other arbirtrary commands from executing on a queue which use the lazy_queue property but likely requires a decent amount of work to support in the current POC. It also may not be feasible in the current implementation to indicate to the queue which submissions are intended for a graph and which are not without making changes to the PI interface by modifying pi_queue, which seems like it may be required to solve this problem.