One operator has at most one output

[primenumber]

What this patch does to fix the issue.

In blueoil/converter, some operators has multiple outputs, but current code base cannot hold multiple outputs correctly.
This PR divides Split operator into Slice operators, Unique operator into UniqueValue and UniqueIndex operators.

Link to any relevant issues or pull requests.

Workaround solution for #1061 (see #1061 (comment) ).

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[tkng]

Other than a very small change, LGTM.

[tkng]

Thanks!
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[tkng]

/ready

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