Add support for Pythonic search space to SimpleSampler

[toshihikoyanase]

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Motivation

Currently, SimpleSampler does not provide Pythonic search space support (a.k.a., define-by-run), but it is one of Optuna's key features (See https://optuna.org).
This PR tries to add support for Pythonic search space while keeping the simplicity of the current SimpleSampler.
Additionally, it renames SimpleSampler to SimpleBaseSampler to clarify that it is an abstract class.

Description of the changes

• Accept search_space=None in SimpleSampler.__init__. In this case, search space is estimated like Optuna's built-in samplers
• Add a default implementation of infer search space with IntersectionSearchSpace
• Add a default implementation of independent sampling with RandomSampler
• Update existing user-defined samplers

[y0z]

Confirmed.