Issue with SemiValue batching and parallelization

[AnesBenmerzoug]

While working on PR #341, I realized that there is a bug in the batching feature of semivalues when using n_jobs > 1.
The results are almost the same but not exactly the same.

[AnesBenmerzoug]

@kosmitive could you have a look at this if you have time?

[kosmitive]

Yes, it might be related to the parallelization as due to parallel processing, the order in which numbers arrive might be prone to a racing condition. I recall this occurs only for semivalues as we break down the calculation to single marginals. Or do you think it might be a different problem?

For the desribed problem, we could introduce a order resolver on the main thread, but at the cost of blowing up RAM on average of about N/2*C where N is the number of processes and C cost per process.

[kosmitive]

@AnesBenmerzoug Here is the reference which was made in the tests

pyDVL/tests/value/test_semivalues.py

Lines 228 to 229 in 7c003be

	# Occasionally, batch_2 arrives before batch_1, so rtol isn't always 0.
	check_values(result_single_batch, result_multi_batch, rtol=1e-4)

. Do we want to go for a order resolution object for the batches?

[AnesBenmerzoug]

@kosmitive I changed that test to use a deterministic scoring method coming from a toy game, so the order of batches shouldn't have an effect on the final result.

[schroedk]

Potentially resolved by #558