If a tf.VariableScope is explicitly passes as an argument to

rnn.bidirectional_rnn/rnn.bidirectional_dynamic_rnn, its name is
concatenated to "_FW"/"_BW" and not the actual scope -- which woulg
raise a TypeError: unsupported operand type(s) for +: 'VariableScope'
and 'str'.
Change: 128624621