Add option to specify an initialization function for `'loky'` and `'multiprocessing'` backends

[shwina]

Closes https://github.com/joblib/joblib/issues/381

This PR adds the ability to specify an initialization function that is run once per worker process when using the 'loky' and 'multiprocessing' backends.

Usage:

Parallel(initializer=my_func, initargs=(x, y, z))
# or
with parallel_config(initializer=my_func, initargs=(x, y, z)):
    ...

[shwina]

Hi @ogrisel - wondering if you have any thoughts on how to test the implementation here correctly?

[bdice]

Hi @ogrisel, this feature request has come up a couple times recently, including in a StackOverflow question: https://stackoverflow.com/questions/78642680/using-load-ext-cudf-pandas-throws-attributeerror

Would it be possible to work with a joblib developer to identify how to move forward with tests?

[SJ-Innovation]

Hi, just wondering is theres been any behind-the-scenes motion on this, as this would be a REALLY handy piece of functionality to have for pre-seeding workers with large, constant, but procedurally generated/loaded datasets at the start of a large number of jobs.