[Core] Refactor set_identity_function.h

[loumalouomega]

📝 Description

This PR refactors the SetIdentityFunction class in the kratos/containers/set_identity_function.h file. The changes include modifying the class definition and providing additional comments and documentation. The SetIdentityFunction class is a functor that serves as the identity function, allowing objects to be used as keys in sets or other containers that require comparison. The class provides two overloaded operator() functions, one for non-const objects and another for const objects, both of which return the input object as it is. The purpose of this refactoring is to improve code clarity and maintainability. The PR also adds appropriate pragma once directives and organizes the include statements.

🆕 Changelog

• Clean up set_identity_function.h