[core] [GraphRegex] Add ? quantifier and related issue

Right now, the "?" quantifier (meaning 0 or 1) is not supported, but it would be very useful in practice (much more than * actually).

Also, the following query does not seem to work:

Pad*->Conv->ReLU*

See unit test Test_example.cpp to reproduce the issue.

Same issue with fuseMulAdd, when changing the query from MatMul -> Add to MatMul -> Add*, the first set of nodes in the unit test is not matched!