Fix various using namespace issues in executorch (#5464)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5464

Summary
#buildsonlynotests - No runtime effects!
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ig-no-test - Doesn't require Instagram testing.

Reviewed By: palmje

Differential Revision: D62953540

fbshipit-source-id: 2ca3cd05710e8c7c7ef33c4fef7a9005ed0c617c