[NVIDIA TF] Register BF16 CudnnRnn kernel
@reedwm @kaixih Could you please review? Thanks!

@kaixih Do you know if CudnnRnn has unit tests? Yes, the `cudnn_rnn_ops_test.cc` is one C++ test which needs bazel build. And `cudnn_rnn_grad.py` is one python test.
bfloat16 datatype is not yet supported by CudnnRnn, so for now these kernels will just cast to float.

Fixes https://github.com/tensorflow/tensorflow/issues/59728
