Adopt runtime::FunctionRef in thread_parallel_interface.h and thread_parallel.h

This saves a *ton* of size by no longer requiring callers to create std::function.

Test Plan: bash test/build_optimized_size_test.sh && size test/size_test_all_optimized_ops

before:
```
ExecuTorch with no ops binary size, unstripped:
-rwxr-xr-x  1 swolchok  staff  153928 Apr 24 13:11 cmake-out/test/size_test
ExecuTorch with portable ops binary size, unstripped:
-rwxr-xr-x  1 swolchok  staff  2150960 Apr 24 13:11 cmake-out/test/size_test_all_ops
ExecuTorch with optimized ops binary size, unstripped:
-rwxr-xr-x  1 swolchok  staff  11182136 Apr 24 13:11 cmake-out/test/size_test_all_optimized_ops

__TEXT	__DATA	__OBJC	others	dec	hex
5341184	98304	0	4300800000	4306239488	100ac0000
```

after:
```
ExecuTorch with no ops binary size, unstripped:
-rwxr-xr-x  1 swolchok  staff  153928 Apr 24 13:07 cmake-out/test/size_test
ExecuTorch with portable ops binary size, unstripped:
-rwxr-xr-x  1 swolchok  staff  2150960 Apr 24 13:07 cmake-out/test/size_test_all_ops
ExecuTorch with optimized ops binary size, unstripped:
-rwxr-xr-x  1 swolchok  staff  5927336 Apr 24 13:07 cmake-out/test/size_test_all_optimized_ops

__TEXT	__DATA	__OBJC	others	dec	hex
4505600	98304	0	4296376320	4300980224	1005bc000
````
`__TEXT` size improvement is 835584 bytes, or 15%.


ghstack-source-id: 2482ad7
ghstack-comment-id: 2828755250
Pull-Request-resolved: #10442

Author: swolchok

extension/threadpool/thread_parallel.cpp

@@ -54,7 +54,7 @@ bool parallel_for(
     const int64_t begin,
     const int64_t end,
     const int64_t grain_size,
-    const std::function<void(int64_t, int64_t)>& f) {
+    runtime::FunctionRef<void(int64_t, int64_t)> f) {
   ET_CHECK_OR_RETURN_FALSE(
       begin >= 0 && end >= 0 && end >= begin,
       "begin = %" PRId64 ", end = %" PRId64,

runtime/kernel/thread_parallel_interface.h

@@ -13,6 +13,7 @@
 
 #include <c10/util/irange.h>
 #include <executorch/runtime/core/error.h>
+#include <executorch/runtime/core/function_ref.h>
 #include <executorch/runtime/platform/assert.h>
 
 namespace executorch {
@@ -69,7 +70,7 @@ bool parallel_for(
     const int64_t begin,
     const int64_t end,
     const int64_t grain_size,
-    const std::function<void(int64_t, int64_t)>& f);
+    runtime::FunctionRef<void(int64_t, int64_t)> f);
 
 int64_t get_thread_num();