Default object is null. in functions.dropout()

[vbkaisetsu]

Example code:

In [1]: from primitiv import Device, Graph, functions, devices, Shape

In [2]: dev = devices.Naive()

In [3]: Device.set_default(dev)

In [4]: g = Graph()

In [5]: Graph.set_default(g)

In [6]: node = functions.ones([2,2])

In [7]: node2 = functions.dropout(node, 0.5, True)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-7-a7a723262cfe> in <module>()
----> 1 node2 = functions.dropout(node, 0.5, True)

/home/koichi/developments/primitiv/primitiv-python/primitiv/_function.pyx in primitiv._function.functions.dropout()

RuntimeError: /home/koichi/developments/primitiv/primitiv-python/primitiv-core/primitiv/mixins.h: 121: Default object is null.

dropout internally calls random::bernoulli, which can take a Graph object, without setting a graph, so random::bernoulli tries to use the default graph. However, primitiv-python no longer use C++'s set_default (#41).