Add allowed_transitions in __init__.py of crf mod.

Fix ImportError: from allennlp.modules.conditional_random_field import allowed_transitions ImportError: cannot import name 'allowed_transitions' from 'allennlp.modules.conditional_random_field'

Fixes #5695.

Changes proposed in this pull request:

• Fix ImportError of allowed_transitions

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I finished this in #5706. Thank you!