Quantized RNNCell modules (#15469)

Quantized RNNCell modules (#15469)

Summary:
Similarly to https://github.com/pytorch/pytorch/pull/13777, we apply post-processing quantization to RNN cell modules (`RNNCell`, `LSTMCell`, and `GRUCell`).

A further follow-up PR will involve quantizing the full `RNN`, `GRU`, and `LSTM` modules. This depends on those modules being scriptable as part of the standard library scripting effort, though. Note that infrastructure in this pr such as `gather_quantized_params` is currently unused but should be used in the future when we can port over the full RNN modules.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15469

Differential Revision: D13545802

Pulled By: jamesr66a

fbshipit-source-id: ad3b694517842893ea619438e9f5e88fd7b96510