Reset inputs after exported for out_grad

[disktnk]

• use BatchNorm
• set train=False, output_grad=True

then got

inputs = (array([[[[0., 0., 0., ..., 0., 0., 0.],
         [0., 0., 0., ..., 0., 0., 0.],
         [0., 0., 0., ..., 0., 0., 0....., ..., 0., 0., 0.],
         [0., 0., 0., ..., 0., 0., 0.],
         [0., 0., 0., ..., 0., 0., 0.]]]], dtype=float32))

    def forward(self, inputs):
        self.retain_inputs((0, 1, 2, 4))
        x, gamma, mean, var, gy = inputs
        expander = self.expander
        xp = backend.get_array_module(x)

        if self.inv_std is None or self.inv_var is None:
            self.inv_var = xp.reciprocal(var + self.eps)
            self.inv_std = xp.sqrt(self.inv_var, dtype=self.inv_var.dtype)

        self.gamma_over_std = gamma * self.inv_std
>       x_hat = _x_hat(x, mean[expander], self.inv_std[expander])
E       TypeError: 'NoneType' object is not subscriptable

This PR fixed unexepected retained variable node by restored hooked inputs.

[codecov-io]

Codecov Report

Merging #238 into master will increase coverage by 0.02%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   91.37%   91.39%   +0.02%     
==========================================
  Files          24       24              
  Lines        1611     1615       +4     
==========================================
+ Hits         1472     1476       +4     
  Misses        139      139

Impacted Files	Coverage Δ
onnx_chainer/export.py	93.28% <100%> (+0.1%)	:arrow_up:

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[disktnk]

/test

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