feat: add Transformer encoder, decoder, and decoder_layer;

[WenjieDu]

What does this PR do?

Fixing #171

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 5910317953

• 57 of 90 (63.33%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.8%) to 83.383%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/modules/self_attention.py	38	71	53.52%

Files with Coverage Reduction	New Missed Lines	%
pypots/modules/self_attention.py	1	73.85%
pypots/cli/doc.py	3	82.28%

Totals
Change from base Build 5899489151:	-0.8%
Covered Lines:	3106
Relevant Lines:	3725

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💛 - Coveralls