[MPI] Unidist hungs when finishing work

[Retribution98]

Unidist returns a result, but one of the workers cannot finish the job.

import unidist
import time

@unidist.remote
def g(number):
    time.sleep(1)
    return number**2

@unidist.remote
def f():
    results = []
    for i in range(10):
        results.append(unidist.get(g.remote(i)))
    print('The f faster than faster_fn')
    return results

@unidist.remote
def faster_fn():
    time.sleep(0.1)
    print('The faster_fn faster than f')

unidist.init()
ref1 = f.remote()
ref2 = faster_fn.remote()
unidist.wait([ref1, ref2], num_returns=1)

The reproducer should be running with UNIDIST_CPUS=5