Emulate GPU without sleep

[spitzcor]

Let me preface my comments with a declaration that this is just my understanding how things work. I haven't inspected or even run this code myself.

To keep the focus on IO and to make it possible to test without GPUs, the benchmark has no requirement for a real GPU. This makes sense, however, there is a sleep() involved to simulate the GPU computation time, which seems unnecessary. It appears that there is no overlapping IO and GPU emulation, so it could be removed. Otherwise, it is just a waste of time, because while it may be necessary to simulate the IO-compute flow of a real code, it isn’t necessary to simulate the wallclock runtime. Even if there is some IO and GPU emulation overlap, it seems that long-duration sleeps could be short-circuited once the IO for the stage was complete. Then, the metrics for compute, IO time, fill time, and samples/sec can be calculated as if the same wallclock time was spent, if necessary.