Save training logs into tensorboard files, enable setting num_workers of DataLoader, and add typing annotations

[WenjieDu]

In this PR, we mainly

1. enable saving training logs into tensorboard files;
2. enable setting num_workers of DataLoader;
3. add typing annotations of functions;

[coveralls]

Pull Request Test Coverage Report for Build 4645441456

• 351 of 391 (89.77%) changed or added relevant lines in 26 files are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 84.932%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/classification/base.py	12	13	92.31%
pypots/classification/brits.py	23	24	95.83%
pypots/classification/grud.py	21	22	95.45%
pypots/classification/raindrop.py	22	23	95.65%
pypots/clustering/vader.py	36	37	97.3%
pypots/data/utils.py	9	10	90.0%
pypots/imputation/base.py	13	14	92.86%
pypots/imputation/saits.py	26	28	92.86%
pypots/clustering/base.py	10	13	76.92%
pypots/forecasting/base.py	10	13	76.92%

Files with Coverage Reduction	New Missed Lines	%
pypots/clustering/base.py	1	34.62%
pypots/forecasting/base.py	1	32.47%
pypots/forecasting/bttf.py	1	97.4%
pypots/imputation/locf.py	1	90.57%

Totals
Change from base Build 4635191765:	-0.1%
Covered Lines:	2700
Relevant Lines:	3179

---

💛 - Coveralls