A unified API for model loading

Currently, for loading different models in Tunix we need to load the model and its params_loader in a very hard python specific module loading (e.g. from tunix.models.qwen3.params import create_model_from_safe_tensors or from tunix.models.qwen3 import model as model_lib)

I think it is based on best practices that creating a higher level abstraction like AutoModel for model creation and loading it weights must be beneficial and unifying all the models under the same abstract AutoModel