code: def forget_xy(t):
    
    shape = (t.shape[0], None, None, t.shape[3])
    return tf.placeholder_with_default(t, shape)
docs: Ignore sizes of dimensions (1, 2) of a 4d tensor in shape inference.
  
    This allows using smaller input sizes, which create an invalid graph at higher
    layers (for example because a spatial dimension becomes smaller than a conv
    filter) when we only use early parts of it.