Dynamic model class loading

[comaniac]

This PR attempts to make the model loading process more systematic and self-contained. Currently when addinga new model, we have to explicitly import the model entry class in model_runnter.load_model function. This introduces two drawbacks:

1. model_runner.py cannot focus on the changes of "model runner".
2. Developers have to change 2 files when adding a new model. The change inmodel_runner.py is easy to be missed and may result in conflicts.

sglang.srt does not follow the standard Python package structure (i.e., leveraging __init__.py to construct module hierarchy), so it's not straightforward to use a model registry with decorator. In this PR, I tried to dynamically scan all model files under models, and load their entry classes. The major drawback of this approach is that every model file has to have EntryClass alias to be scanned. IMHO it should be relatively easy for developers to follow.

Please share your thoughts.