fp16 is broken
run run_glue.py with the parameter of --fp16, and return error:
```
RuntimeError: Incoming model is an instance of torch.nn.parallel.DistributedDataParallel. Parallel wrappers should only be applied to the model(s) AFTER
the model(s) have been returned from amp.initialize.
```

i find the reason is the wrong order of `amp.initialize` and `model = torch.nn.DataParallel`, [iIf DDP wrapping occurs before amp.initialize, amp.initialize will raise an error](https://github.com/NVIDIA/apex/blob/master/examples/imagenet/README.md), and it worked after i change the order
Indeed, thanks! Fixed in master