Transpose2d, Concatenate2d, and up to 3 Clones for io_stream
This PR has 3 independent, minor changes (I can split them into smaller PRs if needed):
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Transpose2d for io_stream
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Concatenate2d for io_stream
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Clone streams up to 3 times (instead of just up to 2 times) -
Add unit test for all 3
These features are needed, for example in this interaction network model: https://gist.github.com/jmduarte/9ea95df65feac29f9663c47a439e2d7e