Defer FX graph stringification until DEBUG logging is enabled.

[micwill755]

Summary

• Stop building the full FX graph string on every logger.debug call. logger.debug("…" + str(gm.graph)) and f-string dumps run Graph.__str__ even at INFO.
• Pass gm.graph as a %s argument so Python logging formats it only when DEBUG is actually enabled.
• Covers the compile hot path (compile / refit / torch.compile backend and the lowering passes that dumped the graph after each rewrite).

PR 7 of the compile-startup stack; based on #4597.

Test plan

• tests/py/dynamo/lowering/test_lazy_graph_printing.py (source scan for eager str(gm.graph) in dynamo debug logs; constant_fold does not stringify at INFO and still does at DEBUG)
• Klein/Flux not re-run: estimated ~40–55 ms at INFO/WARNING from ~6 dumps × ~8–9 ms; 0 at DEBUG

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}