Make a consistent policy for instantiation of storage classes and publicization of private storage accompanying it

blocked by https://github.com/optuna/optuna/issues/2938

Motivation

Currently, CachedStorage and InMemoryStorage are private to users, and it is not expected that users will instantiate them directly using their constructors. However, the handling of instances is inconsistent in the following points:

• RDBStorage and RedisStorage are public and are expected to be instantiated by users. Otherwise, these two classes cannot specify their functionality by the string argument.
• Since trial.storage is public, we can access any instance of storage via the public API.

It is necessary to determine a consistent policy for instantiation of the all storage classes, and to determine their accesses (public/private) of CachedStorage and InMemoryStorage based on it.

Description

The policy will be decided in consideration of the following points. Also, the changes have been merged.

• Consistent policy for instantiation of the all storage classes
• Based on that, their accesses (public/private) of CachedStorage and InMemoryStorage have been decided.

TODOs

• Decide consistent policy for instantiation of the all storage classes
• Their accesses (public/private) of CachedStorage and InMemoryStorage have been decided.
• Implementation

[keisuke-umezawa]

@himkt You are right! But, to close this issue, I need to summarize the situation beforehand. Wait a moment for it.

[HideakiImamura]

Thanks for the summarization. I think this Issue is fine to close. Please let me do that.