Change training pybind extension_module linkage to static (#9809)

Summary: Fixes #9576. Use `extension_module_static` in building
`_training_lib`.

Test Plan: Rely on unit test, also did a manual install in editable
mode:

```bash
./install_executorch.sh --pybind training -e
python -c "from executorch.extension.training.pybindings._training_lib import get_sgd_optimizer"
```

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