Treat keyboard interruption during training as a warning, and update the docs

[WenjieDu]

What does this PR do?

1. treat keyboard interruption during training as a warning;
2. update the docs;

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).
• 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 8998459338

Details

• 1 of 41 (2.44%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 82.464%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/classification/base.py	0	4	0.0%
pypots/clustering/base.py	0	4	0.0%
pypots/clustering/crli/model.py	0	4	0.0%
pypots/clustering/vader/model.py	0	4	0.0%
pypots/forecasting/base.py	0	4	0.0%
pypots/forecasting/csdi/model.py	0	4	0.0%
pypots/imputation/base.py	0	4	0.0%
pypots/imputation/csdi/model.py	0	4	0.0%
pypots/imputation/gpvae/model.py	0	4	0.0%
pypots/imputation/usgan/model.py	0	4	0.0%

Totals
Change from base Build 8980856697:	-0.2%
Covered Lines:	8192
Relevant Lines:	9934

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💛 - Coveralls