Fix protobuf errors when using system protobuf
This seems to fail API compat test. Can you run the following, please?

```
bazel run tensorflow/tools/api/tests:api_compatibility_test -- --update_goldens True
```

Thank you
```
File "/home/conda/lib/python3.10/site-packages/tensorflow/python/framework/meta_graph.py", line 557, in create_meta_graph_def
    meta_graph_def.graph_def.MergeFrom(graph_def)
TypeError: Parameter to MergeFrom() must be instance of same class: expected tensorflow.GraphDef got tensorflow.GraphDef.
```

This error happened in TF 2.4 and 2.5 and was fixed. However, it appeared again in TF 2.15, as `from tensorflow.python import pywrap_tensorflow` was removed in 719f6da42226319a9348f55b86d3103881bae0ee. See previous issues and fixes:
- https://github.com/tensorflow/tensorflow/issues/42596
- bad835f9af3f1e1e04f1ea69db902add1b5076f9
- https://github.com/tensorflow/tensorflow/issues/50545
- https://github.com/tensorflow/tensorflow/pull/51450

This PR adds `from tensorflow.python import pywrap_tensorflow` to `tensorflow/__init__.py` (generated by `api_template.__init__.py`), and it fixes the error when I test it. I am not sure whether there is a better place to add this line. (it seems `tensorflow/python/__init__.py` does not have any import)