Implementation of DummyInferenceEngine

[rahat2134]

This PR addresses issue #325 by implementing a DummyInferenceEngine for testing purposes.

Changes

• Created DummyInferenceEngine class in exo/inference/dummy_inference_engine.py
• Modified exo/inference/inference_engine.py to include the dummy engine option
• Updated exo/main.py to support selection of the dummy engine
• Added basic testing for the DummyInferenceEngine

Features

• Simulates inference without loading a real model
• Generates random token outputs
• Simulates latency using asyncio.sleep
• Fully asynchronous implementation
• Simulates shard loading

Testing

• Added basic test in main.py to verify DummyInferenceEngine functionality
• Manually tested with python main.py --inference-engine dummy

Feedback and suggestions for improvement are welcome!