Add invoke loop into GPU delegate for TFLite GPU benchmarking to exclude CPU-GPU memory copy & sync time from total latency.
Add invoke loop into GPU delegate for TFLite GPU benchmarking to exclude CPU-GPU memory copy & sync time from total latency.

This is to be used only by TFLite benchmarker and only enabled by "--copt=-DTFLITE_GPU_ENABLE_INVOKE_LOOP" flag.
