python frontend: support nested loops

[asraa]

https://github.com/google/heir/pull/1307 adds support for single loops with at most one iter_arg, so add support for nested loops (propogate the inner body's iter_arg through the loop nest) and generalize to an arbitrary number of iter_args.

[github-actions[bot]]

This issue has 1 outstanding TODOs:

• frontend/heir/mlir_emitter.py:802: support nested loops.

This comment was autogenerated by todo-backlinks

[asraa]

Blocked on https://github.com/llvm/llvm-project/issues/129901