[FR] Build mock models for developers to assist debugging

[blueset]

Is your feature request related to a problem? Please describe.
Model developers may face problems like slow startup or hard issue isolation while using existing models for debugging.

Describe the solution you'd like
Make mock models that exposes an interface (potentially HTTP) to cover all basic features defined for the model type.

Describe alternatives you've considered
Other ideas are welcome too.

Additional context
Inspired by discussion with Cat Baron in Support Group.