title: PyTorch flatten doesn't maintain batch size
body: <p>In Keras, using the <code>Flatten()</code> layer retains the batch size. For eg, if the input shape to Flatten is <code>(32, 100, 100)</code>, in <code>Keras</code> output of Flatten is <code>(32, 10000)</code>, but in PyTorch it is <code>320000</code>. Why is it so?</p>