What does this PR do?

Update docs and 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).
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• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.