Potential New Feature: allowing users to input customized initial pipelines

After thoroughly reviewing the well-structured and logically clear code of TPOT, we have identified that the construction of pipelines in the initial population is random. It seems like a natural idea to allow users to provide some well-defined initial pipelines to TPOT. This approach has the potential to enhance the algorithm's performance and reduce the evolutionary time.

Additionally, we have observed some related issues ([#1107](https://github.com/EpistasisLab/tpot/issues/1107)) and pull requests ([#502](https://github.com/EpistasisLab/tpot/pull/502)). Regrettably, these updates were not implemented optimally. We are highly motivated to work on developing this new feature and have made some initial progress. However, it's worth noting that TPOT's framework is complex, and we are not yet fully familiar with it. For instance, we haven't yet deciphered the function of **_self.tree_structure_** in _/tpot/base.py_.

Considering your expertise with TPOT's inner workings, we would greatly appreciate your assistance in resolving issues that may arise during our development efforts. We believe that collaborating in this manner will not only help us comprehend TPOT's development principles but also contribute to the overall advancement of the project.

Thank you very much for considering our request and for your continued contributions to TPOT. We look forward to working together to further enhance this remarkable tool. 🤝