- Add build target for jax_plugins/ and jax_plugins/cuda for bazel test.
- Update jax_plugins/cuda/__init__.py to fallback to local `.so` file path.
- Add a flag --//jax:build_cuda_plugin to control whether to link in local cuda plugin.
The following command will test with cuda plugin:
```
bazel test tests:python_callback_test_gpu --test_output=all --test_filter=PythonCallbackTest.test_send_zero_dim_arrays_pure --config=tensorflow_testing_rbe_linux --config=rbe_linux_cuda12.2_nvcc_py3.9 --//jax:build_cuda_plugin=false
```
Default behavior (without `--//jax:build_cuda_plugin=false`) remains unchanged.
PiperOrigin-RevId: 582728477
With this change, existing plugin discovery mechanism can discover local plugins without pip install.
Update jax_plugins/cuda/__init__.py to return without registering the plugin if the .so file does not exist.
PiperOrigin-RevId: 582431300
