fix: add FlattenVector transformer

[s0nicboOm]

1. Fix saving of stateless transformer
2. Introduce FlattenVector

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (146ec00) 93.35% compared to head (99e284a) 93.48%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #344 +/- ## ========================================== + Coverage 93.35% 93.48% +0.12% ========================================== Files 83 83 Lines 3840 3853 +13 Branches 310 311 +1 ========================================== + Hits 3585 3602 +17 + Misses 198 195 -3 + Partials 57 56 -1 ```

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