[perf][1/5] Replace IValue::toString()->string() with IValue::toStringRef() (#85437)
Summary: `IValue::toString()` creates a `new c10::intrusive_ptr` (like `std::shared_ptr`) and `->string()` immediately accesses it, creating an atomic reference increment/decrement. We can skip both of these operations by calling `IValue::toStringRef()`.
Test Plan: CI
Reviewed By: jaybean-dev
Differential Revision: D39605242
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85437
Approved by: https://github.com/jfix71
