mypy: Fix return issues

[wochinge]

Maxime recently enabled mypy support for the first couple of issues. In our current setup we ignore certain typing issues as there are currently too many to tackle them all at once.

We should instead tackle them step by step. Related issue. Parent issue.

Current number of issues with this type: 18

Definition of Done:

• enable mypy checks for return
• all existing return issues are fixed

[joejuzl]

Found 98 errors in 40 files (checked 262 source files) 😱

[wochinge]

Oh dear. I can help and we do divide and conquer?

[joejuzl]

I'll do a first pass, and see how time consuming it - then maybe we can divide some of the more complex areas.

[joejuzl]

I've got it down to 54 errors. These are the files that are left:

rasa/shared/nlu/training_data/message.py
rasa/utils/tensorflow/model_data_utils.py
rasa/utils/tensorflow/models.py
rasa/shared/nlu/training_data/formats/rasa_yaml.py
rasa/nlu/components.py
rasa/shared/core/training_data/structures.py
rasa/shared/importers/importer.py
rasa/shared/core/generator.py
rasa/core/nlg/callback.py
rasa/nlu/model.py
rasa/shared/core/training_data/story_reader/yaml_story_reader.py
rasa/nlu/test.py
rasa/core/featurizers/tracker_featurizers.py
rasa/shared/core/training_data/story_writer/yaml_story_writer.py
rasa/nlu/selectors/response_selector.py
rasa/nlu/featurizers/sparse_featurizer/lexical_syntactic_featurizer.py
rasa/core/test.py
rasa/core/policies/rule_policy.py
rasa/core/policies/form_policy.py
rasa/core/actions/forms.py
rasa/server.py
rasa/core/training/interactive.py
rasa/utils/tensorflow/environment.py

If you want to give me a hand, you could have a look at the first half? (i.e. to yaml_story_reader.py)
Some of them are a bit tricky btw 😄

[wochinge]

Sure 👍🏻 What's your branch?
Btw, I've also fixed some other error codes in here #9043

[joejuzl]

branch is 8242-mypy_return_issues, but probably makes sense to do it on a different branch for now.

[wochinge]

I'm doing my changes on this PR #9054 (fixed message and model_data_utils so far)