Response Selector in Cross Validation

[dakshvar22]

Proposed changes:

• Added evaluation of response selection as part of cross validation pipeline.
• Mostly mimics how evaluation for intent classification is done.

Status (please check what you already did):

• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[wochinge]

Left a couple of comments, especially tests would be great.
Also, I'm getting this when running rasa test nlu --cross-validation on the scaffold project:

Traceback (most recent call last):
  File "/Users/tobias/.pyenv/versions/3.7/bin/rasa", line 11, in <module>
    load_entry_point('rasa', 'console_scripts', 'rasa')()
  File "/Users/tobias/Workspace/stack/rasa/__main__.py", line 76, in main
    cmdline_arguments.func(cmdline_arguments)
  File "/Users/tobias/Workspace/stack/rasa/cli/test.py", line 143, in test_nlu
    perform_nlu_cross_validation(config, nlu_data, output, vars(args))
  File "/Users/tobias/Workspace/stack/rasa/test.py", line 206, in perform_nlu_cross_validation
    data, folds, nlu_config, output, **kwargs
  File "/Users/tobias/Workspace/stack/rasa/nlu/test.py", line 1271, in cross_validate
    train,
  File "/Users/tobias/Workspace/stack/rasa/nlu/test.py", line 1178, in combine_result
    ) = compute_metrics(interpreter, data)
  File "/Users/tobias/Workspace/stack/rasa/nlu/test.py", line 1359, in compute_metrics
    intent_results, "intent_target", "intent_prediction"
  File "/Users/tobias/Workspace/stack/rasa/nlu/test.py", line 1481, in _compute_metrics
    results, target_key, target_prediction
ValueError: not enough values to unpack (expected 2, got 0)

[dakshvar22]

@wochinge Added tests and types. I couldn't reproduce the error that you got. What dataset and config did you use?

[wochinge]

Good to go for me! Nice to see some love for the evaluation feature 🥇
Re the error with too few training data: create an issue and put in the maintenance board

[wochinge]

optional: test if the report was written