Make the self-attention operator replaceable in Transformer

[WenjieDu]

What does this PR do?

1. fixing Make Transformer layers more flexible #333;

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 8517997717

Details

• 19 of 22 (86.36%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 80.64%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/transformer/attention.py	10	11	90.91%
pypots/nn/modules/transformer/layers.py	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/nn/modules/transformer/layers.py	2	83.67%

Totals
Change from base Build 8510512186:	0.03%
Covered Lines:	6598
Relevant Lines:	8182

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💛 - Coveralls