适用lambda进行传参：
x = tf.placeholder(tf.float32)
y = tf.placeholder(tf.float32)
z = tf.placeholder(tf.float32)
def fn1(a, b):
return tf.mul(a, b)
def fn2(a, b):
return tf.add(a, b)
pred = tf.placeholder(tf.bool)
result = tf.cond(pred, lambda: fn1(x, y), lambda: fn2(y, z))
