Replace `std::stringstream` with `std::string` for Shader names (#2964)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2964

## Context

Some research into efficient string concatenation suggests that streams in C++ are not quite efficient. The best way to concatenate strings seems to be creating a `std::string` and reserving sufficient capacity for the `std::string`. This diff deprecates the usage of `std::stringstream` when constructing kernel names in favor of using `std::string` directly.

Reviewed By: copyrightly

Differential Revision: D55951475

fbshipit-source-id: a1a584669e80984b85d11b7d6d4f7593290e562b