FEAT: generator support normal function

[codingl2k1]

What do these changes do?

• If a normal function is decorated by xo.generator, then the function will be run by to_threads.
• Regardless of whether a function is decorated by xo.generator returns a generator or an async generator or a value, the function should works as expected.

Related issue number

Fixes #xxxx

Check code requirements

• [ ] tests added / passed (if needed)
• [ ] Ensure all linting tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a442021) 89.12% compared to head (5d346f6) 89.02%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #83 +/- ## ========================================== - Coverage 89.12% 89.02% -0.10% ========================================== Files 48 48 Lines 4010 4010 Branches 764 764 ========================================== - Hits 3574 3570 -4 - Misses 351 355 +4 Partials 85 85 ``` | [Flag](https://app.codecov.io/gh/xorbitsai/xoscar/pull/83/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=xorbitsai) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/xorbitsai/xoscar/pull/83/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=xorbitsai) | `88.85% <ø> (-0.10%)` | :arrow_down: | 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=xorbitsai#carryforward-flags-in-the-pull-request-comment) to find out more.

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