Fix/no lambda function in add_encoders

[madtoinou]

Fixes #1843.

Summary

• Raise an error when a lambda function is used in the add_encoders parameter so that model pickling works as intended when saving the model.
• Updated documentation and notebooks accordingly

Other Information

• Since all the examples in the documentation (notebook included) used lambda function, this will probably cause issue for a lot of user.
• Instead of an exception, we could eventually just display a warning so that users can eventually use another pickling module that user lambda functions.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (6096968) 93.84% compared to head (9695dff) 93.84%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1957      +/-   ##
==========================================
- Coverage   93.84%   93.84%   -0.01%     
==========================================
  Files         132      132              
  Lines       12731    12721      -10     
==========================================
- Hits        11948    11938      -10     
  Misses        783      783              

Files Changed	Coverage Δ
darts/dataprocessing/encoders/encoders.py	97.61% <100.00%> (+0.03%)	⬆️

... and 6 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[dennisbader]

Looks good, thanks @madtoinou 🚀
Only thing left is to update the add_encoders model docs that still use the lambda functions.

[dennisbader]

Nice, thanks @madtoinou 🚀
Last thing to do is add it to the changelog, as a breaking change).