Stateful Dataflow runner?

Hi,

I'm trying to use python portable DataflowRunner with a [BagStateSpec]([https://beam.apache.org/releases/pydoc/2.6.0/apache_beam.transforms.userstate.html)]. Though I encounter followiung issue:
```

Traceback (most recent call last):
File "/Users/leopold/.pyenv/versions/3.6.0/lib/python3.6/runpy.py",
line 193, in _run_module_as_main
"__main__", mod_spec)
File "/Users/leopold/.pyenv/versions/3.6.0/lib/python3.6/runpy.py",
line 85, in _run_code
exec(code, run_globals)
File "/Users/leopold/workspace/BenchmarkListingStreaming/listing_beam_pipeline/test_runner.py",
line 49, in
run()
File "/Users/leopold/workspace/BenchmarkListingStreaming/listing_beam_pipeline/test_runner.py",
line 44, in run
| 'write to file' >> WriteToText(known_args.output)
File "/Users/leopold/.pyenv/versions/BenchmarkListingStreaming/lib/python3.6/site-packages/apache_beam/pipeline.py",
line 481, in __exit__
self.run().wait_until_finish()
File "/Users/leopold/.pyenv/versions/BenchmarkListingStreaming/lib/python3.6/site-packages/apache_beam/runners/dataflow/dataflow_runner.py",
line 1449, in wait_until_finish
(self.state, getattr(self._runner, 'last_error_msg', None)), self)
apache_beam.runners.dataflow.dataflow_runner.DataflowRuntimeException:
Dataflow pipeline failed. State: FAILED, Error:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/dataflow_worker/batchworker.py",
line 648, in do_work
work_executor.execute()
File "/usr/local/lib/python3.6/site-packages/dataflow_worker/executor.py",
line 176, in execute
op.start()
File "apache_beam/runners/worker/operations.py", line 649, in
apache_beam.runners.worker.operations.DoOperation.start
File "apache_beam/runners/worker/operations.py",
line 651, in apache_beam.runners.worker.operations.DoOperation.start
File "apache_beam/runners/worker/operations.py",
line 652, in apache_beam.runners.worker.operations.DoOperation.start
File "apache_beam/runners/worker/operations.py",
line 261, in apache_beam.runners.worker.operations.Operation.start
File "apache_beam/runners/worker/operations.py",
line 266, in apache_beam.runners.worker.operations.Operation.start
File "apache_beam/runners/worker/operations.py",
line 597, in apache_beam.runners.worker.operations.DoOperation.setup
File "apache_beam/runners/worker/operations.py",
line 636, in apache_beam.runners.worker.operations.DoOperation.setup
File "apache_beam/runners/common.py",
line 866, in apache_beam.runners.common.DoFnRunner.__init__
Exception: Requested execution of a stateful
DoFn, but no user state context is available. This likely means that the current runner does not support
the execution of stateful DoFns.

```

I've also seen this issue in stackoverflow

[https://stackoverflow.com/questions/55413690/does-google-dataflow-support-stateful-pipelines-developed-with-python-sdk](https://stackoverflow.com/questions/55413690/does-google-dataflow-support-stateful-pipelines-developed-with-python-sdk)

Do you have any idea/ETA when this feature will be available with beam?

Thanks!

Imported from Jira [BEAM-9655](https://issues.apache.org/jira/browse/BEAM-9655). Original Jira may contain additional context.
Reported by: leopold.boudard.