Update on "[Executorch][threadpool] Add "private" method to reset num threads in threadpool"

This diff introduces new API to reset threadpool. This will be short-lived
features. It is needed atm because we dont have a) a way to use only a subset of
threads to distribute work in the threadpool and b) API to create threadpool of
custom size and pass such a threadpool to backends like delegtes, cusotm ops,
optimized cpu op lib etc.

Differential Revision: [D54766070](https://our.internmc.facebook.com/intern/diff/D54766070/)

[ghstack-poisoned]

Author: kimishpatel

backends/xnnpack/threadpool/threadpool.cpp

@@ -45,8 +45,8 @@ size_t ThreadPool::get_thread_count() const {
 }
 
 bool ThreadPool::_unsafe_reset_threadpool(uint32_t new_thread_count) {
-  // No need to do anything if the count is same
-  if (new_thread_count == get_thread_count()) {
+  // No need to do anything if the count is same or 0
+  if (new_thread_count == get_thread_count() || new_thread_count == 0) {
     return true;
   }