Add split_entities_config to TEDPolicy for e2e

[koernerfelicia]

Rasa version:
2.2.2
Python version:
3.7.2
Operating system (windows, osx, ...):
OSX

Issue:

TEDPolicy needs a split_entities_config value, to mirror CRFEntityExtractor and DIET

When we call convert_predictions_into_entities, the value of split_entities_config is None, which is only a problem if we then call EntityExtractor._check_is_single_entity. I think this is somewhat rare, which is why the bug went unnoticed so far.

TEDPolicy should have its own value for this, with the same default as DIET, and this parameter should be configurable in the same way.