fix predict with multiple models

[jmoralez]

Description

255 introduced a bug where the original series values weren't backed up correctly, so when using multiple models the first model would get the correct values but the second one would get the predictions from the first one instead of the original values and so on.

Fixes #289

Checklist:

• [x] This PR has a meaningful title and a clear description.
• [x] The tests pass.
• [x] All linting tasks pass.
• [x] The notebooks are clean.

[review-notebook-app[bot]]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB