Enhanced usage of `append` instead of using `extend`

[Anselmoo]

Checklist

• [x] I think this refactoring is useful for everyone (if it's too specific, consider a custom rule)
• [x] I have checked there are no similar issues suggesting the same thing

Description

Please check

https://github.com/keras-team/keras-tuner/pull/780#discussion_r1043471881 https://github.com/keras-team/keras-tuner/pull/780#discussion_r1043461352

[ruancomelli]

Great idea, I have added it to our roadmap!