Add automatic hyperparameter tuning

[neilmcguigan]

🚀 Feature Request

it would be cool if you added an automatic hyperparameter tuning feature, as per https://facebook.github.io/prophet/docs/diagnostics.html#hyperparameter-tuning

🔈 Motivation

this would create better forecasts

[github-actions]

Hello @neilmcguigan, thank you for your interest in our work!

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