Multitask models should be across tasks

[WenjieDu]

1. Feature description

For multi-task methods, they should be across tasks, rather than having separate implementation for each task.

2. Motivation

This can help simplify the code.

3. Your contribution

Will give a thought about how to do the code refactor.

[WenjieDu]

We have modularized all NN models and included them in pypots.nn.modules package. These modules can be reused in the implementations of multitask models.