[quant] Add save/load state_dict to quantized dynamic RNNs (#39105)
Summary:
Previously dynamic LSTM modules weren't able to save/load from state_dict since PackedParameter used in RNNs isn't serializable from python
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39105
Test Plan: python test/test_quantization.py TestSerialization
Reviewed By: jerryzh168
Differential Revision: D21752256
Pulled By: supriyar
fbshipit-source-id: ef82cf21ce21a3a1304d147ed0da538c639f952d
