Add allowed_transitions in allennlp_light.modules.conditional_random_field.__init__.py

[tchayintr]

I am working a class that inherits ConditionalRandomField class from allennlp_light.modules.conditional_random_field.

I think it would be more flexible if the allowed_transitions function is allowed to be imported like from allennlp_light.modules.conditional_random_field import allowed_transitions.

I did go through the original https://github.com/allenai/allennlp, they already did it.

allenai/allennlp#5695
allenai/allennlp#5697
allenai/allennlp#5706

Although this will be a small patch, I think there will be many parts we still can update and improve from time to time.

If you agree to update, I can PR this small change.