• clamps concat axis before use (otherwise valid fusions where axis is negative won't happen)
• generalizes the axis requirement from channels to any axis as long as all channels preceding the concat axis are singleton

I haven't fixed the comments. Not sure how to rewrite them.

For the list of models you find here:
number of concat fusions before: 60
number of concat fusions after: 72

List of fusions before and after: https://gist.github.com/YashasSamaga/94449832c25f2993cfc8ffc0a8bdf618

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