Add comment to explain rnn bias vectors (#15843)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15843
RNN/LSTMs only need one bias vector, but our implementation uses two to be compatible with CuDNN. This diff adds a comment to explain this.
Reviewed By: ezyang
Differential Revision: D13602365
fbshipit-source-id: eef5bd9383d9f241dc0ef0472f753b4a44cc19b5
