Want a single-objective version of RGPE and TSTR

[nabenabe0928]

Motivation

I want a single-objective version of RGPE and TSTR from this repository.

These algorithms are well-known practical meta-learning GP-BO methods.
As they will be very useful for benchmarking purposes, I would like to have them in OptunaHub.

Description

The implementation here is a multi-objective version and uses BoTorch.

At least, I would like it to be a single-objective version.
Optionally, it would be great if we could remove the BoTorch dependency using the _gp module in Optuna.

Alternatives (optional)

No response

Additional context (optional)

No response