Fix hidden_layer size for one-directional decoder (#99)

* Fix hidden_layer size for one-directional decoder

Hidden layer size of the decoder was given `hidden_size * 2 if bidirectional else 1`, resulting in a dimensionality error for non-bidirectional decoders.
Changed `1` to `hidden_size`.

examples/sample.py

@@ -100,7 +100,7 @@ def len_filter(example):
         bidirectional = True
         encoder = EncoderRNN(len(src.vocab), max_len, hidden_size,
                              bidirectional=bidirectional, variable_lengths=True)
-        decoder = DecoderRNN(len(tgt.vocab), max_len, hidden_size * 2 if bidirectional else 1,
+        decoder = DecoderRNN(len(tgt.vocab), max_len, hidden_size * 2 if bidirectional else hidden_size,
                              dropout_p=0.2, use_attention=True, bidirectional=bidirectional,
                              eos_id=tgt.eos_id, sos_id=tgt.sos_id)
         seq2seq = Seq2seq(encoder, decoder)