Walking nested graph rewriter
Implementation of WalkingNestedGraphRewriter which can apply node_rewriter for nodes of nested graphs (Scan and OpFromGraph). Close #529.
Initially I placed WalkingNestedGraphRewriter in pytensor/graph/rewriting/basic.py but faced with cyclic import issue caused by OpFromGraph, so I moved it into separate module. Also not sure about tests, may be we need add more scenarios or use another approach.
Also this changes should allow us to simplify implementation of default moment for CustomSymbolicDist from https://github.com/pymc-devs/pymc/pull/6873
@ricardoV94 could you please take a look on this PR when you will have some time?
@ricardoV94 just friendly reminder about PR )