Allow RNN decoders to tie embedding and projection weights

[okuchaiev]

RNNDecoderWithAttention and BeamSearchRNNDecoderWithAttention classes should be able to use transpose of elf._dec_emb_w in the self._output_projection_layer. This will probably require changing self._output_projection_layer to a function which does simple matmul, instead of using tf.layers.Dense