RemoveIdentityOps does not correctly handle identity operations following fork-nodes

[iksnagreb]

This problem reported over at FINN belongs here, see Xilinx/finn#878 (comment).

RemoveIdentityOps seems to be broken if the identity operation directly follows a fork-node. It seemingly rewires the fork-node output only into that branch containing the identity op, disconnecting the others. Currently this happens when using packed input projections in multi-head scaled dot-product attention. I will try to fix this soon, probably only requires an additional test added to the remove_node_and_rewire utility function.