Created by: gfwm2013

修改while_loop的英文文档，添加body返回值type和length和loop_vars不相同时的报错，并修改了单测文件。 如下两种情况

def body(i):
    i = 1 + 1
    return [i, i]

def body(i, ten):
    i = 1 + 1
    return i

程序会报错:

ValueError: body in while_loop should return the same arity (length and structure) and types as loop_vars