chainerx.array is not picklable

It is often convenient if we can pickle chainerx arrays when we save/restore/communicate them.

>>> import pickle
>>> import chainerx as chx
>>> a = chx.array(10)
>>> a
array(10, shape=(), dtype=int64, device='native:0')
>>> pickle.dumps
<built-in function dumps>
>>> pickle.dumps(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: return_value_policy = copy, but the object is non-copyable!

[niboshi]

Introducing a Python wrapper class (#7983) does seem to fix the problem.

arr = chainerx.array([1, 2], chainerx.float32, device='cuda:0')
s = pickle.dumps(arr)

Now this variant works with the patch.

[emcastillo]

The problem seems to be this py::return_value_policy::reference, probably is setting something in the python side of the wrapper the first time the corresponding python object is created.

A simple solution to this problem is to return the name of the device when serializing, and reconstruct the device from the name upon deserialization. I quickly tested this and worked. But if we can find a way to make the C++ to be directly pickable it will be better.