Improve models structure

[lolipopshock]

In this update, we aim to improve the structure of the modeling folder to provide better support for building models of different backends.

In the new model folder, we provide the following files as the template to implement models for specific backend:

models
├─ base_catalogue.py
└─ base_layoutmodel.py # contains the template class `BaseLayoutModel` for building new model classes. 

And for the specific model folder, it contains the following files

specific_model_folder/
├─ __init__.py
├─ catalogue.py # specifies the pre-trained model information, need to register `PathManager` from `base_catalogue.py`
└─ layoutmodel.py # contains the model class for the specific backend.