[DOCS] Missing Import for KeepColumns in Instruction Backtranslation

[Hassaan-Qaisar]

Which page or section is this issue related to?

https://github.com/argilla-io/distilabel/blob/main/docs/sections/pipeline_samples/papers/instruction_backtranslation.md

What are you documenting, or what change are you making in the documentation?

The issue here is that the KeepColumns class is referenced in your pipeline definition but it hasn't been imported. This omission causes a NameError when running the code because Python doesn't recognize the KeepColumns symbol.