update ProcessPoolExecutor docs

[mathuvu]

Types of changes

• [x] Docs change / refactoring / dependency upgrade
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context / Related issue

ThreadPoolExecutor use only one thread for all task in async due to GIL lock (one interpreter = one thread). For CPU-bound function ProcessPoolExecutor circumvent this issue.

How Has This Been Tested (if it applies)

import nevergrad as ng
from concurrent import futures
import time
import multiprocessing as mp

from nevergrad.optimization import optimizerlib

def func(x):
    tmp = 1
    for k in range(4_000_000):#Garbage calculation
        tmp += 1
        tmp -= 1
    return sum((x-1)**2)*tmp
 parametrization = ng.p.Array(shape=(10,))

start = time.time()
budget = 100

optim = optimizerlib.OnePlusOne(parametrization=parametrization, budget=budget, num_workers=10)
# recom = optim.minimize(func, executor=futures.ThreadPoolExecutor(max_workers=optim.num_workers))
recom = optim.minimize(func, executor=futures.ProcessPoolExecutor(max_workers=optim.num_workers, mp_context=mp.get_context('fork')))# Spawn mode raise an error, so it will not work on windows.

end = time.time()
print(end - start)

Checklist

• [x] The documentation is up-to-date with the changes I made.
• [x] I have read the CONTRIBUTING document and completed the CLA (see CLA).
• [ ] All tests passed, and additional code has been covered with new tests.

[teytaud]

I think this PR is super important but complicated for CircleCI because we are not allowed to do multiprocessing.... But we need to change this because of https://stackoverflow.com/questions/58919495/why-is-threadpoolexecutor-slower-than-for-loop)

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