[quant] Enable jit tracing on quantizable LSTM (resubmission) (#64638)

[quant] Enable jit tracing on quantizable LSTM (resubmission) (#64638)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64638

The quantizable LSTM didn't support jit tracing because it had several non taceable paths. We sacrifice some of the user experience to enable the tracing.
The main UX feature removed is a user-friendly message when trying to access the backwards path in a bidirectional LSTM: When the bidirectional flag is False, we used to throw a nice error message when the user tried accessing backwards weights. Now the message is default (removed properties).

Test Plan: `buck test mode/dev //caffe2/test:quantization -- test_custom_module_lstm`

Reviewed By: HDCharles

Differential Revision: D30803753

fbshipit-source-id: a639955a96cee22538d9436f1c952a5d121f50f9