test_split_crazy_sdf broken in python presubmit. 'DataInputOperation' object has no attribute 'index'

[https://builds.apache.org/job/beam_PreCommit_Python_Phrase/331/testReport/junit/apache_beam.runners.portability.fn_api_runner_test/FnApiRunnerSplitTest/test_split_crazy_sdf_2/](https://builds.apache.org/job/beam_PreCommit_Python_Phrase/331/testReport/junit/apache_beam.runners.portability.fn_api_runner_test/FnApiRunnerSplitTest/test_split_crazy_sdf_2/)

Traceback (most recent call last): File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Phrase/src/sdks/python/test-suites/tox/py35/build/srcs/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 157, in _execute response = task() File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Phrase/src/sdks/python/test-suites/tox/py35/build/srcs/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 216, in lambda: self.progress_worker.do_instruction(request), request) File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Phrase/src/sdks/python/test-suites/tox/py35/build/srcs/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 312, in do_instruction request.instruction_id) File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Phrase/src/sdks/python/test-suites/tox/py35/build/srcs/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 354, in process_bundle_split process_bundle_split=processor.try_split(request)) File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Phrase/src/sdks/python/test-suites/tox/py35/build/srcs/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 588, in try_split desired_split.estimated_input_elements) File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Phrase/src/sdks/python/test-suites/tox/py35/build/srcs/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 144, in try_split if total_buffer_size < self.index **** 1: AttributeError: 'DataInputOperation' object has no attribute 'index'

Imported from Jira [BEAM-7006](https://issues.apache.org/jira/browse/BEAM-7006). Original Jira may contain additional context.
Reported by: ajamato@google.com.