[Dy2stat] Fix the bug that loop_body_func may return single element · Pull Request !59 · PaddlePaddle/Paddle

### 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.

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