Fix NameError and AttributeError of value_at_risk sampler

[msakai]

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Motivation

Sorry, my previous PR #320 has bugs that causes the following runtime errors.

• NameError: name 'Callable' is not defined. Did you mean: 'callable'?
  • Callable is used in cast(Callable[[gp.GPRegressor], torch.Tensor], ...), but is imported only during type checking.
• AttributeError: module 'optuna._gp' has no attribute 'acqf'
  • optuna._gp.acqf is used in cast(optuna._gp.acqf.BaseAcquisitionFunc, acqf) and it seems we need to import optuna._gp.acqf explicitly.

Description of the changes

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[not522]

LGTM!