Eliminate c10::guts::to_string (#108480)
This PR replace c10::guts::to_string with std::to_string. The major part of changes is using void* as optimizer state key since string is used only for serialization and using pointers as hashing keys is more efficient than a string.
Some other guts functions in the affected source files are also replaced.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108480
Approved by: https://github.com/Skylion007
