Add output type check

[disktnk]

If the target model returns tuple of ndarray or other value besides chainer.Variable, exporter cannot make computational graph using variable backword. For example, the error message reported in #128 is probably caused by outputs, the target model returns a tuple, and the tuple includes ndarray. On this code https://github.com/chainer/chainercv/blob/v0.12.0/chainercv/links/model/faster_rcnn/faster_rcnn.py#L159 , roi and roi_indices is ndarray, not chainer.Variable

This PR prevent from exporting when flat_output includes not chainer.Variable type.

[shinh]

Looks great, thanks!

[codecov-io]

Codecov Report

Merging #145 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage      85%   85.29%   +0.29%     
==========================================
  Files          22       22              
  Lines        1120     1122       +2     
==========================================
+ Hits          952      957       +5     
+ Misses        168      165       -3

Impacted Files	Coverage Δ
onnx_chainer/export.py	92.51% <100%> (+1.24%)	⬆️

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[disktnk]

ChainerCI, test this please

[pfn-ci-bot]

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