fix insertion before a node with multiple inputs + support additional broadcasting

This PR has a few related fixes

• fix insertion before a node with multiple inputs
• correctly insert broadcast stream before a node with multiple inputs (e.g. Add or Concatenate), e.g. see figure below. The current code would behave incorrectly and replace the wrong input (always the first input) for the following layer... this would result in an additional output incorrectly being inferred
• don't insert broadcast before concatenate (figure below incorrectly does this)

To do

• Add a pytest

@nickshey