Replace deprecated np.asscalar() with np.ndarray.item()

[tfujiwar]

What this patch does to fix the issue.

To Suppress several hundreds of lines of warning messages.

https://buildkite.com/blueoil/dlk-test/builds/2640#e08ef65c-f4e8-494c-b2ed-e242af5106e1

blueoil/converter/core/operators.py:1450
...
blueoil/converter/core/operators.py:1450
  /home/blueoil/blueoil/converter/core/operators.py:1450: DeprecationWarning: np.asscalar(a) is deprecated since NumPy v1.16, use a.item() instead
    return np.asscalar(self._input_ops['Z'].data)

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[iizukak]

OA

[iizukak]

/ready

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