disable inner parallel for global avg pool as normally they are small (#9487)
* Using cost model's thread count rather than max number of threads when
parallel tasks.
* according to perf test result, decrease parallel on channels.
* Seems no use on parallel channels for qavg_pool according several models, remove it.
* Revert "Using cost model's thread count rather than max number of threads when"
This reverts commit 5fa47cd5b5ddbaa4e5ef97ccbc53200324379544.