title: Keras Conv1D with dilation and "causal" padding isn't reducing the size of the sequence
body: <p>In Keras, if you run</p>
  
  <pre><code>x = Input(shape=(seq_length, dim))
  y = Conv1D(num_filts, 2, dilation_rate=8, padding="causal")(x)
  print(y.shape)
  </code></pre>
  
  <p>Then the shape printed out will be <code>(?, seq_length, dim)</code>, which means the sequence length wasn't changed at all. But isn't this incorrect? Because the dilation makes the effective kernel width of the convolution 9, shouldn't the sequence length of the output be decremented by 8 because the causal kernel needs 9 values to perform its dot product and so doesn't output on the first 8 samples in the sequence? Is this a problem with Keras?</p>