Allow concat_arrays to be pickable

[emcastillo]

This tries to fix a bug that happens when using MultiprocessParallelUpdater with forkserver or spawn as the start mode.

There is an error because the multiprocessing module tries to pickle the converter function and this one has lost its complete import route from the func_name or __name__ attribute due being decorated with converter.

Essentially I avoid the name classing in the module by not using the decorator.

I tried to solve this using functools.wrap but the case its trickier than it seems at first.

[emcastillo]

This is an snippet reproducing the issue

import pickle


class A:
    def __init__(self, func):
        self.func = func


def decorator():
    def wrap(func):
        return A(func)
    return wrap


@decorator()
def test():
    pass


class ThingWithBoundedFunc:
    def __init__(self, func=test):
        self.func = test


if __name__ == '__main__':
    obj = ThingWithBoundedFunc()
    pickle.dumps(obj)  # Fails
    pickle.dumps(test)  # Fails

either pickling obj or just the test function fails
seems like test is missing the import route on it's __name__ attribute when it is wrapped in the A class. Resulting in the following error:

_pickle.PicklingError: Can't pickle <function test at 0x7f7eabe32488>: it's not the same object as __main__.test

functools.wraps couldn't be used to solve this, as it is not a function wrapping a function but, an object.

[toslunar]

The current approach LGTM.

I tried to solve this using functools.wrap but the case its trickier than it seems at first.

Maybe because #7489 introduced the check if isinstance(converter, Converter).

chainer/chainer/dataset/convert.py

Line 128 in a2804df

if isinstance(converter, Converter):

[toslunar]

Could you fix this error?

Warning, treated as error:
/home/travis/virtualenv/python3.5.6/lib/python3.5/site-packages/chainer/dataset/convert.py:docstring of chainer.dataset.converter:41:Unknown directive type "notes".

.. notes:: Converters using this decorator can't be pickled,

[toslunar]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit cf70171, target branch master) failed with status FAILURE.

[emcastillo]

Finally travis passed! Sorry about it

[emcastillo]

PTAL, Thanks!!

[toslunar]

Jenkins, test this please.

[toslunar]

Thanks. LGTM.

[chainer-ci]

Jenkins CI test (for commit 807cb5f, target branch master) succeeded!

[emcastillo]

I think failures are not related. thanks

[toslunar]

TravisCI: tests/chainermn_tests/extensions_tests/test_multi_node_snapshot.py failed on MNIST download (urllib.error.HTTPError: HTTP Error 403: Forbidden).
Seems unrelated to the PR.

[toslunar]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit c6a9a9c, target branch master) failed with status FAILURE.

[emcastillo]

Same 403,
I can access the url from my local env though 😥

[chainer-ci]

@toslunar This pull-request is marked as st:test-and-merge, but there were no activities for the last 3 days. Could you check?

[toslunar]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit c6a9a9c, target branch master) succeeded!