style: use postponed evaluation of annotations

[XuehaiPan]

Description

Describe your changes in detail.

Use postponed evaluation of annotations with from __future__ import annotations.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds core functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [X] I have read the CONTRIBUTION guide. (required)
• [X] My change requires a change to the documentation.
• [X] I have updated the tests accordingly. (required for a bug fix or a new feature)
• [X] I have updated the documentation accordingly.
• [X] I have reformatted the code using make format. (required)
• [X] I have checked the code using make lint. (required)
• [X] I have ensured make test pass. (required)

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (78e23bc) compared to base (b064f75). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #28 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 346 347 +1 ========================================= + Hits 346 347 +1 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `100.00% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/metaopt/optree/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt) | Coverage Δ | | |---|---|---| | [optree/ops.py](https://codecov.io/gh/metaopt/optree/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt#diff-b3B0cmVlL29wcy5weQ==) | `100.00% <100.00%> (ø)` | | | [optree/registry.py](https://codecov.io/gh/metaopt/optree/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt#diff-b3B0cmVlL3JlZ2lzdHJ5LnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt)

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