Build `fit` `evaluate` function within the model class

[cshjin]

• [x] To build a standard interface within the model class
• [x] No need to create an additional train function outside the class
• [x] Let the class keep track of some metrics / checkpoints during training / validation stages

[cshjin]

Check the update in https://github.com/cshjin/swmp_ml/blob/master/py_script/model.py