Fix device synchronization in local_cupy benchmark (#518)

The way the code was written would pass the client's device to each worker and synchronize on that, which is wrong. This was catched due to recent changes to Distributed, where the following was raised:

```python
Task exception was never retrieved
future: <Task finished coro=<Scheduler.broadcast.<locals>.send_message() done, defined at /datasets/pentschev/miniconda3/envs/gdf/lib/python3.7/site-packages/distributed/scheduler.py:4965> exception=Exception("unhashable type: 'cupy.cuda.device.Device'")>
Traceback (most recent call last):
  File "/datasets/pentschev/miniconda3/envs/gdf/lib/python3.7/site-packages/distributed/scheduler.py", line 4969, in send_message
    resp = await send_recv(comm, close=True, serializers=serializers, **msg)
  File "/datasets/pentschev/miniconda3/envs/gdf/lib/python3.7/site-packages/distributed/core.py", line 662, in send_recv
    raise Exception(response["text"])
Exception: unhashable type: 'cupy.cuda.device.Device'
```

Authors:
  - Peter Andreas Entschev (@pentschev)

Approvers:
  - Benjamin Zaitlen (@quasiben)

URL: #518

dask_cuda/benchmarks/local_cupy.py

@@ -75,7 +75,7 @@ async def _run(client, args):
         res = client.compute(func(*func_args))
         await client.gather(res)
         if args.type == "gpu":
-            await client.run(xp.cuda.Device().synchronize)
+            await client.run(lambda xp: xp.cuda.Device().synchronize(), xp)
         took = clock() - t1
 
     return {