Isolation of optuna integration modules.

Optuna is now isolating optuna.integration module to optuna-integration.
The following is a list of integration modules and their priorities.

Do not migrate low-priority module because it will have some complex dependencies on tests and others. (Migration of low-priority modules would be done by Committers.)

Migration procedure

Following is the transfer procedure, please follow these instructions

On the Optuna Integration side:

1. Clone the optuna-integration forked repository
   git clone git@github.com:{YOUR-FOR}/optuna-integration.git
2. Add the Optuna master branch to the local upstream.
   git remote add upstream https://github.com/optuna/optuna.git
3. Create the new working branch
   git checkout -b YOUR-BRANCH-NAME.
4. Check out the relevant file for integration purposes.
   git fetch upstream and git checkout upstream/master -- <path/to/file>
   • The desired files are as follows
     • optuna/integration/<module>
     • tests/integration_tests/<module>
   • Move the file paths from optuna/integration/<module> to optuna_integration/<module> and tests/integration_tests/<module> to tests/<module>.
   • Create a commit for each file, with a commit message that reads

     "Fetched from https://github.com/optuna/optuna/commit/<commit-hash>".

   • <commit-hash> can be obtained using git rev-list upstream/master -- <path/to/file> | head -n 1.
5. In the desired files, replace optuna._imports with optuna_integration._imports.
6. Update the required dependencies in pyproject.toml. Especially, please add the corresponding third party library in the all section of the pyproject.toml.
7. Modify the CI workflow file if necessary. Check the Optuna's CI workflow file of integration, i.e., .github/workflows/tests-integration.yml. If you find the any specific treatment for the modified integration, please add it to the optuna-integration's CI workflow file, i.e., .github/workflows/tests.yml.
8. Update the imports in tests to use optuna_integration instead of optuna.integration.
9. Add the new module to docs/source/reference/index.rst (from here).
10. Add the new module to the relevant section in README.md.
11. Change all optuna.integration sentences to optuna_integration (if they exist).

On the Optuna side:

1. Import the package that was migrated from optuna_integration and pass its contents to __all__ magic variable.
   • For example.

from optuna_integration.chainer import ChainerPruningExtension


__all__ = ["ChainerPruningExtension"]

1. Remove the relevant files from test/integration and related imports from test/integration/test_integration.py.
2. Remove any relevant tests from pytest tests -m "integration" --ignore in .github/workflows/tests-integration.yml.
3. Remove the relevant documentation from docs/source/reference/integration.rst.
4. Remove the integration dependencies from setup.py and pyproject.toml, taking care not to remove dependencies that are still needed.
   • Do not remove the relevant imports from _import_structure variable in optuna/integration/__init__.py, as this would render the from optuna.integration import ... idiom unusable.
5. Transfer any relevant issues from Optuna to Optuna Integration.
6. Request that the author of any relevant PRs transfer them from Optuna to Optuna Integration.

These changes should be merged into optuna-integration at first, then merged into optuna.

On the Optuna exmaple side:

1. Change the corresponding integration's ci yaml to install optuna-integration.