PyTorch adopts C++20 rvalue overload to eliminate string copying in stringstream

A recent commit in the PyTorch repository (PR #186552) introduces a performance optimization by adopting the C++20 rvalue overload for `std::stringstream::str()`. Before C++20, calling `str()` on a `stringstream` always returned a copy of the internal string buffer, which could be expensive for large strings or frequent calls. The new rvalue overload, introduced in C++20, allows the caller to move the internal buffer directly, avoiding the copy entirely.

The commit, titled "Use rvalue overload for stringstream str," makes a mechanical find-and-replace across the PyTorch codebase, assisted by OpenAI's Codex AI tool. The change was reviewed and approved by Meta engineer Nikita Shulga (malfet). By switching to the rvalue overload, any code that calls `str()` on a temporary `stringstream` object or that no longer needs the stream afterward can now steal the string instead of copying it. This is particularly beneficial in performance-critical paths like logging, error messages, and serialization where strings are built and then immediately used.

This optimization aligns with modern C++ best practices and demonstrates how AI-assisted refactoring can quickly propagate low-level improvements across large codebases. The impact is measurable in reduced CPU cycles and memory allocations, especially for applications that generate many diagnostic strings or process large amounts of text. For PyTorch users, this means slightly faster training loops, data loading, and debugging output, though the gains are incremental per call.