Add Gradio App and local inference example

[mingxin-zheng]

This PR adds

• Inference environment setup via makefile
• Inference example with gradio
• A basic framework to add expert models into the real-time inference
• Instruction how to set up the VLM locally

To use the expert models, NV API keys are needed for now. In the future, I will provide local mode for the expert models as well.

[holgerroth]

This is great!