code: def retrieve_seq_length_op2(data):
      
      return tf.reduce_sum(tf.cast(tf.greater(data, tf.zeros_like(data)), tf.int32), 1)
docs: An op to compute the length of a sequence, from input shape of [batch_size, n_step(max)],
      it can be used when the features of padding (on right hand side) are all zeros.
  
      Parameters
      -----------
      data : tensor
          [batch_size, n_step(max)] with zero padding on right hand side.
  
      Examples
      --------
      >>> data = [[1,2,0,0,0],
      ...         [1,2,3,0,0],
      ...         [1,2,6,1,0]]
      >>> o = retrieve_seq_length_op2(data)
      >>> sess = tf.InteractiveSession()
      >>> tl.layers.initialize_global_variables(sess)
      >>> print(o.eval())
      [2 3 4]