A bug in the trainer_misc/communicate.py

The number of input parameters for function `forward` in class `_AllToAll` is not equal to the number returned by `backward`. This will cause the following error:

```
RuntimeError: function _AllToAllBackward returned an incorrect number of gradients (expected 6, got 5)
```

Adding a new `None` as a return parameter to `backward` solves this problem.