[iree-benchmark-module] transfer outputs to host in case of enable_output_processing (#24821)

The enable_output_processing feature added to iree-benchmark_module in
commit a70389bb8b [iree-benchmark-module] support processing outputs
after benchmarking (#24789) did not call
iree_tooling_transfer_variants() to make sure the output buffers are
host accessible. So depending on the device used for benchmarking, the
outputs could be accessed on the host to dump those or to check those.
This PR fixes this by calling iree_tooling_transfer_variants() in case
output processing is enabled. This is the same handling as in
iree-run-module.

Signed-off-by: Stefan Schuermans <schuermans@roofline.ai>