Improve shape function for CudnnRNNParamsSize

In cudnn_rnn_ops.cc, the CudnnRNNParamsSize does not
have restrictions on num_layers, num_units, and input_size,
though they all should be scalars.

This fix adds the shape check of num_layers, num_units, and input_size
for CudnnRNNParamsSize.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>