code: def combine_first_two_dimensions(x):
    
    ret = tf.reshape(x, tf.concat([[-1], common_layers.shape_list(x)[2:]], 0))
    old_shape = x.get_shape().dims
    a, b = old_shape[:2]
    new_shape = [a * b if a and b else None] + old_shape[2:]
    ret.set_shape(new_shape)
    return ret
docs: Reshape x so that the first two dimension become one.
  
    Args:
      x: a Tensor with shape [a, b, ...]
  
    Returns:
      a Tensor with shape [ab, ...]