message: Fix shapes in comments of nmt_with_attention.ipynb
  It is a bit misleading and confusing that the output shape of decoder is currently commented as `(batch_size * max_length, vocab)`. However the correct shape should be `(batch_size * 1, vocab)`, since the input x of GRU layer has shape == `(batch_size, 1, embedding_dim + hidden_size)`.
diff: " # passing the concatenated vector to the GRU\n",
  " output, state = self.gru(x)\n",
  " \n",
  - " # output shape == (batch_size * max_length, hidden_size)\n",
  + " # output shape == (batch_size * 1, hidden_size)\n",
  " output = tf.reshape(output, (-1, output.shape[2]))\n",
  " \n",
  - " # output shape == (batch_size * max_length, vocab)\n",
  + " # output shape == (batch_size * 1, vocab)\n",
  " x = self.fc(output)\n",
  " \n",
  " return x, state, attention_weights\n",
  