[Dy2stat] Fix the bug that loop_body_func may return single element

[zhhsplendid]

PR types

Bug fixes

PR changes

APIs

Describe

Our old loop_body function may return single element when loop_vars just contains only 1 element, which can cause bug. Let's see an example, the origin code is the new testcase TestForwardContainsForLayer of this PR. The translated code would be like below before this PR

def forward(self, x):
    y = paddle.zeros([10, 2, 3])
    z = []
    i = 0

    def for_loop_condition_0(i):
        return i < self.high - self.low

    def for_loop_body_0(i):
        paddle.jit.dy2static.convert_call(z.append)(paddle.jit.dy2static.
            convert_call(y[i].clone)())
        i += 1
        return i # This PR changes `return i` to `return i,` as a tuple

    [i] = paddle.jit.dy2static.convert_while_loop(for_loop_condition_0,
        for_loop_body_0, [i])
    return z

You could notice the for_loop_body_0 returns i, then when the python for loop is running, the loop_vars [i] would be updated to i, which can cause bug.

The key point of this PR is forcing loop_body functions always return tuple.

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[liym27]

LGTM