Include init file for superoffload folder (#7591)
This PR just fixes tiny error for pr
[7559](https://github.com/deepspeedai/DeepSpeed/pull/7559) in the
comment reported error
[here](https://github.com/deepspeedai/DeepSpeed/pull/7559#issuecomment-3329036699).
```
[rank1]: File "/usr/local/lib/python3.11/dist-packages/deepspeed/runtime/engine.py", line 1462, in _configure_optimizer
[rank1]: self.optimizer = self._configure_zero_optimizer(basic_optimizer)
[rank1]: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank1]: File "/usr/local/lib/python3.11/dist-packages/deepspeed/runtime/engine.py", line 1835, in _configure_zero_optimizer
[rank1]: from deepspeed.runtime.superoffload.superoffload_stage3 import SuperOffloadOptimizer_Stage3
[rank1]: ModuleNotFoundError: No module named 'deepspeed.runtime.superoffload'
```
Create `__init__.py` for superoffload folder to avoid import error when
superoffload folder irgnored by pip installation.
---------
Signed-off-by: nguyen599 <pnvmanh2123@gmail.com>
