[Unity] Fix FewShotTuning Failure When Missing Global Symbol

[zxybazh]

Previously the building of PrimFuncs in Few Shot Tuning pass will fail when the function attribute global_symbol is missing. This PR manually adds a global symbol of main for each PrimFunc to tune and the unit tests are modfied accordingly.

Thanks @masahi for identifying this issue.

[tvm-bot]

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

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