Import forgotten pack_weight_bias in rnn.py (#84315)
`pack_weight_bias` is exported in `__all__`, but the actual import was lot during migration in https://github.com/pytorch/pytorch/pull/78714.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84315
Approved by: https://github.com/seemethere, https://github.com/malfet
