make the number of steps of stable diffusion configuration

[freedomtan]

the tflite implementation https://github.com/mlcommons/mobile_app_open/blob/master/mobile_back_tflite/cpp/backend_tflite/stable_diffusion_pipeline.h#L40

supposed, both

• the seed, and
• the number of steps (default to 20)

should be configurable from the app or from the pbtxt (task description, https://github.com/mlcommons/mobile_app_open/blob/master/flutter/assets/tasks.pbtxt).

(let's change the tflite backend number of steps to 20 first).