Fix import error breaking AllReduceRunner._send_error_to_peer()

[borzunov]

This fixes errors like this:

[2021/08/24 19:59:38.543][ERROR][asyncio._run_internal:255] Task exception was never retrieved
future: <Task finished name='Task-2845' coro=<AllReduceRunner._send_error_to_peer() done, defined at /slot/sandbox/nv_tmpfs/d/in/script/0_script_unpacked/hivemind/hivemind/averaging/allreduce.py:232> exception=NameError("name 'afirst' is not defined")>
Traceback (most recent call last):
  File "/slot/sandbox/nv_tmpfs/d/in/script/0_script_unpacked/hivemind/hivemind/averaging/allreduce.py", line 235, in _send_error_to_peer
    await afirst(self._get_peer_stub(peer_id).rpc_aggregate_part(aiter(error)))
NameError: name 'afirst' is not defined

[codecov]

Codecov Report

Merging #360 (22cca24) into master (4890a75) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
+ Coverage   83.45%   83.46%   +0.01%     
==========================================
  Files          66       66              
  Lines        6139     6139              
==========================================
+ Hits         5123     5124       +1     
+ Misses       1016     1015       -1     

Impacted Files	Coverage Δ
hivemind/averaging/allreduce.py	77.70% <100.00%> (ø)
hivemind/averaging/matchmaking.py	83.18% <0.00%> (+0.30%)	⬆️