Created by: iclementine

PR types

Bug fixes

PR changes

APIs

Describe

1. fix a bug in RNN(that it does not initialize the state for the cell correctly). The code to generate default initial state for a cell wrapped in an RNN is moved to paddle.nn.functional.rnn;
2. BiRNN now does not reuse the initial state for forward & backward initial states. It must be a 2-tuple of cell states, or None;
3. add testcases of RNN & BiRNN;
4. update unittests for static graph by using paddle.enable_static().
5. fix typos in docstrings. bias_ih_attr -> bias_hh_attr