Get nodes that match patterns

[jambayk]

The onnx rewriter matches node patterns and replaces them. It would be a useful functionality if the node pattern matcher could be used to get the list of node groups that match patterns.
This could be used to check for existence of patterns, or the user can do things outside of the rewrite rules on the matched nodes.

[justinchuby]

So instead of doing the full replacement, an api to just find matches may be helpful. I was hoping to have something like that the other day too. @gramalingam

[jambayk]

This is optional, but if having a generator as output might be nice too so we can do a fast check for existence of a pattern (return True if pattern exists at least once) without having to go iterator without having to go through the whole model.