Remove OptunaConfig.

[toshihikoyanase]

Optuna's config file is used to specify default storage in $HOME/.optuna.yml. This PR removes it because of the following reasons:

1. it's a bit complicated because it is only used by optuna commands. $HOME/.optuna.yml is ignored if users execute the code by Python interpreter.
2. Other than the default storage, Optuna has more configuration points such as samplers and pruners, and users want a full-featured configuration method as can be seen in Add config dict interface #475.

I'd like to remove the current configuration method and prepare for the discussion to improve Optuna's configuration.

[sile]

LGTM

[g-votte]

Since we no longer need tempfile_config, can't we delete StorageSupplier in this file and use optuna.testing.storage.StorageSupplier instead?

For example, we could modify test_create_study_command as follows:

def test_create_study_command():
    # type: () -> None

    with StorageSupplier('new') as storage:
        storage_url = str(storage.engine.url)

        # Create study.
        command = ['optuna', 'create-study', '--storage', storage_url]
        subprocess.check_call(command)

        # Command output should be in name string format (no-name + UUID).
        study_name = str(subprocess.check_output(command).decode().strip())
        name_re = r'^no-name-[a-f0-9]{8}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{12}$'
        assert re.match(name_re, study_name) is not None

        # study_name should be stored in storage.
        study_id = storage.get_study_id_from_name(study_name)
        assert study_id == 2

[toshihikoyanase]

Thank you for pointing it out. I fixed it based on your example code in this commit 5b13c76.

def test_create_study_command():
    # type: () -> None

    with StorageSupplier('new') as storage:
+       assert isinstance(storage, RDBStorage)
        storage_url = str(storage.engine.url)

I needed to add an assersion that confirms storage is an instance of RDBStorage because optuna.testing.storages.StorageSupplier can returns None or InMemoryStorage.

[g-votte]

LGTM