tests: add data and tesk unit test

[LongxingTan]

Description

This PR ...

Checklist

• Linked issues (if existing)
• Amended changelog.md for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Thank you for joining. Have fun coding!

[codecov]

Codecov Report

❌ Patch coverage is 92.70833% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.32%. Comparing base (93015da) to head (14cc0f5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tfts/data/timeseries.py	96.15%	3 Missing ⚠️
tfts/trainer.py	76.92%	3 Missing ⚠️
tfts/tasks/base.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   81.46%   83.32%   +1.85%     
==========================================
  Files          55       55              
  Lines        4807     4899      +92     
==========================================
+ Hits         3916     4082     +166     
+ Misses        891      817      -74     

Flag	Coverage Δ
cpu	83.32% <92.70%> (+1.85%)	⬆️
unittest	83.32% <92.70%> (+1.85%)	⬆️

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