Add learning-rate schedulers

[WenjieDu]

What does this PR do?

Fixing #194

Before submitting

• 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 6333854690

• 223 of 254 (87.8%) changed or added relevant lines in 16 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 83.856%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/optim/lr_scheduler/exponential_lrs.py	13	15	86.67%
pypots/optim/lr_scheduler/step_lrs.py	14	16	87.5%
pypots/optim/lr_scheduler/constant_lrs.py	18	21	85.71%
pypots/optim/lr_scheduler/multistep_lrs.py	16	19	84.21%
pypots/optim/lr_scheduler/lambda_lrs.py	16	20	80.0%
pypots/optim/lr_scheduler/linear_lrs.py	19	23	82.61%
pypots/optim/lr_scheduler/multiplicative_lrs.py	17	21	80.95%
pypots/optim/lr_scheduler/base.py	66	75	88.0%

Totals
Change from base Build 6308682398:	0.2%
Covered Lines:	3797
Relevant Lines:	4528

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💛 - Coveralls