Create package pypots.optim to separate optimizers from models

[WenjieDu]

What does this PR do?

Fixes #105. Add the package pypots.optim to help separate optimizers from models to:

1. separate the hyperparameters of models and optimizers in PyPOTS, so that users don't have to put all hyperparameters
   in one place, which could result in a mess and be not readable.
2. provide additional functionalities, such as learning rate scheduling, etc.

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 4974224183

• 75 of 81 (92.59%) changed or added relevant lines in 12 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 83.802%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/locf/model.py	2	3	66.67%
pypots/optim/base.py	17	22	77.27%

Files with Coverage Reduction	New Missed Lines	%
pypots/data/base.py	1	79.8%
pypots/classification/base.py	2	79.52%

Totals
Change from base Build 4955240763:	0.1%
Covered Lines:	2830
Relevant Lines:	3377

---

💛 - Coveralls