Retain hook does not support ndarray

[disktnk]

Process of exporting retains references of input value to output temporary values using RetainInputHook. This hook only supports chainer.Variable, so if the target model handles chainer.Chain accepting numpy.ndarray, cause ValueError.

class Model(chainer.Chain):

    def forward(self, x, h):
        return x + h

class ParentModel(chainer.Chain):

    def __init__(self):
        super().__init__()
        self.h = np.array(5, dtype=np.float32)
        with self.init_scope():
            self.m = Model()

    def forward(self, x):
        return self.m(x, self.h)

x  = chainer.Variable(np.array(3, dtype=np.float32))
export(ParentModel(), (x,))

E           ValueError: type <class 'numpy.ndarray'> is not supported to retain input

ref #201