[Unity] Handle extern func calls in static memory planning

[vinx13]

Handling of external func calls in memory planning was missing before, which disabled memory planning for all BYOC use cases. This PR added handling for external func calls, it assumes external func calls via call_packed doesn't retain memory.

cc @MasterJH5574 @tqchen

[tvm-bot]

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

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