[Unity] Enhance CopyWithNewVars utility

[Ubospica]

This PR enhances the relax::CopyWithNewVars util by adding a var_map member to the FuncCopier class.

And the FuncCopier pass is moved to src/relax/transform/utils.h to include it in other C++ source files.

Therefore, we can know the variable mapping from the old function to the new function. To use this you will need to:

// only copying a function
#include <tvm/relax/utils.h>
auto func = CopyWithNewVars(old_func);

// getting the var_map_ while copying the function
#include "src/relax/transform/utils.h"
auto copier = FuncCopier();
auto func = copier.Copy(old_func);
Map<Var, Var> var_map = copier.var_map;

[tvm-bot]

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• cc @quic-sanirudh _{See #10317 for details}

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