Fix import error breaking AllReduceRunner._send_error_to_peer() (#360)

Import afirst() instead of asingle()

hivemind/averaging/allreduce.py

@@ -8,7 +8,7 @@ from hivemind.averaging.partition import AllreduceException, TensorPartContainer
 from hivemind.p2p import P2P, P2PContext, PeerID, ServicerBase, StubBase
 from hivemind.proto import averaging_pb2
 from hivemind.utils import get_logger
-from hivemind.utils.asyncio import achain, aenumerate, aiter, amap_in_executor, anext, asingle
+from hivemind.utils.asyncio import achain, aenumerate, afirst, aiter, amap_in_executor, anext
 from hivemind.utils.compression import deserialize_torch_tensor, serialize_torch_tensor
 
 # flavour types