[Vulkan][Optimize for Mobile] Avoid dereferencing element [0] if the vector is empty (#92918)
Summary:
Avoid dereferencing element [0] if the vector is empty.
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In ```transferInputOutputBackends```, one of the rewrite passes for Vulkan ```optimize_for_mobile```, an out of bounds access happens when trying to insert a backend transfer for an input if that input's ```uses()``` is empty. This diff corrects that issue.
Test Plan:
Run tests
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Phabricator + CI Tests
Reviewed By: SS-JIA
Differential Revision: D41296037
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92918
Approved by: https://github.com/SS-JIA, https://github.com/kirklandsign
