Add annotation for distributions

[himkt]

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Reference Issues/PRs

N/A

What does this implement/fix? Explain your changes.

distributions in test_search_space.py is not compatible with the type annotation of crate_trial.
We observe mypy failure like following when a check is run with Optuna v3.0.0b0.

> mypy python_tests optuna_dashboard                                                                                                                                                                 2022-04-24 16:30:44
python_tests/test_search_space.py:40: error: Argument "distributions" to "create_trial" has incompatible type "Dict[str, UniformDistribution]"; expected "Optional[Dict[str, BaseDistribution]]"
python_tests/test_search_space.py:40: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
python_tests/test_search_space.py:40: note: Consider using "Mapping" instead, which is covariant in the value type
python_tests/test_search_space.py:71: error: Argument "distributions" to "create_trial" has incompatible type "Dict[str, UniformDistribution]"; expected "Optional[Dict[str, BaseDistribution]]"
python_tests/test_search_space.py:71: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
python_tests/test_search_space.py:71: note: Consider using "Mapping" instead, which is covariant in the value type
python_tests/test_search_space.py:108: error: Argument "distributions" to "create_trial" has incompatible type "Dict[str, UniformDistribution]"; expected "Optional[Dict[str, BaseDistribution]]"
python_tests/test_search_space.py:108: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
python_tests/test_search_space.py:108: note: Consider using "Mapping" instead, which is covariant in the value type
Found 3 errors in 1 file (checked 15 source files)

distributions has a type dict[str, UniformDistribution] that is not compatible with Dict[str, BaseDistribution]. It means the type is not valid in the current implementation even if CIs are all green in the latest master branch.

Optuna v2.10.0 unintentionally hides the information because @experimental decorator crushes a type hint of a method. Optuna v3.0.0 makes create_trial stable (i.e. removes @experimental) and exposes type information to third-party libraries correctly. This is why we observe the mypy failure by upgrading the Optuna.

https://github.com/optuna/optuna/blob/v2.10.0/optuna/trial/_frozen.py#L505-L516

v2.10.0	v3.0.0b0

This PR adds type hints List[Dict[str, BaseDistribution]] for distributions, which is compatible with Optional[Dict[str, BaseDistribution]] of create_trial.

[c-bata]

$ mypy python_tests
Success: no issues found in 6 source files

LGTM after passed all CI tests!!