[tune] remove dependency on gpy

[matthewdeng]

Description

Replace GPy with scikit-learn's GaussianProcessRegressor in the PB2 (Population Based Bandits) scheduler.

Additional information

• Rewrote TV_SquaredExp kernel to implement sklearn's Kernel interface instead of GPy's Kern
• Replaced GPy.models.GPRegression with sklearn.gaussian_process.GaussianProcessRegressor
• Removed gpy from tune-test-requirements.txt
• Updated documentation to only require scikit-learn (which is already a Ray dependency)

Testing

Re-ran existing PB2 tests in est_trial_scheduler_pbt.py to validate change.