Separate optimizers and models

[WenjieDu]

1. Feature description

To make the framework more usable, we should separate models and optimizers, for example:

saits=SAITS(
n_steps=48,
n_features=37,
n_layers=3,
...
optimizer=pypots.optim.Adam(),
...
)

2. Motivation

Such a design can give users more options, and make PyPOTS framework more powerful. For example, we can add more functionalities into pypots.optim.Optimizer classes, like lr scheduler.

3. Your contribution

Will make a PR to implement it.