[onnxruntime_perf_test] Fix custom_allocator_ destruction order. (#24136)
Move the allocator data member declaration before the `Ort::Value`
container data members that might use the allocator so that the
`Ort::Value` containers will be destroyed first.
`custom_allocator_` may be used as the allocator for the `Ort::Value`s
in `test_inputs_` and `outputs_`. The allocator shouldn't be destroyed
before `Ort::Value`s allocated with it are freed.