Improve annotations in automl and ml modules (#919)

* begin annotation in automl.py and ml.py

* EstimatorSubclass + annotate metric

* review: fixes + setting fit_kwargs as proper Optional

* import from flaml.automl.model (import from flaml.model is deprecated)

* comment n_jobs in train_estimator as well

* better annotation in _compute_with_config_base

Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>

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Co-authored-by: Andrea W <a.ruggerini@ammagamma.com>
Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>