Adding the package pypots/modules, and appending modules of vanilla Transformer

[WenjieDu]

What does this PR do?

Fixing #171 and #172

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 5916043036

• 118 of 156 (75.64%) changed or added relevant lines in 4 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 83.293%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/modules/self_attention.py	96	134	71.64%

Files with Coverage Reduction	New Missed Lines	%
pypots/cli/doc.py	3	82.28%

Totals
Change from base Build 5830808857:	-0.9%
Covered Lines:	3106
Relevant Lines:	3729

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💛 - Coveralls