Created by: zhhsplendid

Remove Read-Only Basic Type Variable in loop_vars

I didn't add test because these is an existing test case:

def while_loop_dyfunc_with_none(x):
    i = fluid.dygraph.to_variable(x)\
        if x is not None \
        else fluid.dygraph.to_variable(x+1)
    flag = 1
    while x < 10:
        i = i + x if flag is not None else x + i
        x = x + 1
    return i

As you can see, flag is removed from loop_vars thus not converted into boolean tensor and the numerical test is still correct.

BTW, notice this PR doesn't add attribute into condition / write consideration because we found we are not handling class well now. I will put it as TODO thing.