Domain should be made available for rasa train nlu command if it's available

[dakshvar22]

Part of #6638

Rasa version: master

Issue:

When issuing the command rasa train nlu, train_nlu is invoked inside rasa/train.py which does not have access to the domain. Since responses key can be defined in the domain file as well which may be needed for the response selector training, the file importer should get access to the domain as well if its available. Once that's done responses will be sync'ed with the NLU training data which was added as part of #6591.

Without this, currently running rasa train nlu with ResponseSelector in the pipeline will lead to warnings such as -

Your training data contains an example 'What is your name?...' for the chitchat/ask_name intent. You either need to add a response phrase or correct the intent for this example in your training data. If you intend to use Response Selector in the pipeline, the training may fail.

because these response templates were defined under the responses key in the domain file.

rasa train command works completely fine.

Command or request that led to error:

rasa train nlu

[wochinge]

fixed by #6913