fix the use of incomplete vector<T> for C++20 compatibilities (#93978)
Avoid referring to std::vector<T> members and constructor/desctructors when T is incomplete.
Referring to incomplete members is [not legal](https://timsong-cpp.github.io/cppwp/n4868/vector#overview-4) according to the C++ standard.
Non-noexcept constructors need access to members' destructors. As of C++20, std::vector's destructor is constexpr and so forcefully requires a complete type for the vector's elements.
These issues cause build errors in newer toolchains under c++20 mode.
Fix them by moving code that needs complete types to a different place where the type is already defined.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93978
Approved by: https://github.com/Skylion007
