Integrate Split and Concat related PRs from Upstream

[iksnagreb]

Would be nice to integrate the following contributions related to support of split and concat operations, which are still not merged into the upstream FINN but required by some of my Transformer-related research, as well as upcoming improvements to streamlining and generalized activation function support:

• Streamlining transformations for Split and Concat nodes Xilinx/finn#1189
• Compiler integration of the Concat operation Xilinx/finn#1193
• Compiler integration of the Split operation Xilinx/finn#1194

In general, we should have a discussion on how/when to integrate PRs from the upstream repository: Do we simply PR across forks whenever we need/want something to be integrated here, or should we try to push the existing upstream PR forward (e.g. by adding a review) so we can get this the "usual way" synchronizing with Xilinx/dev?

[LinusJungemann]

In my opinion, we should do both. If we need something, just PR across forks and in addition add a review to the original PR. The review is done by us anyway, if we integrate it here and it helps the original PR getting integrated, but then we also do not need to wait months until something is in the original dev for us to integrate it.