[Feature] Change study to a property

[chulhongsung]

What does this PR do?

Change optuna.Study to property.

How to test this?

mq_lgb = MQRegressor(
    x=x_data,
    y=y_data,
    alphas=alphas,
)

mq_lgb.train(n_trials=5)
mq_lgb.study

[RektPunk]

Thanks for the contribution. BTW, please install pre-commit so that makes code lint.

[RektPunk]

LGTM

[RektPunk]

Could you replace this line with something like:

Suggested change

	return self._study
	return getattr(self, "_study", None)

This helps prevent calling a variable before it's assigned.