Update on "Add c10::irange to ExecuTorch"

irange is a header-only utility that is both more readable than the usual way to write for loops and also solves -Wsign-compare issues. I've previously vetted that it generates the same assembly as a regular for loop.

Differential Revision: [D69817195](https://our.internmc.facebook.com/intern/diff/D69817195/)

[ghstack-poisoned]

Author: Github Executorch

runtime/core/portable_type/c10/README.md

@@ -1,7 +1,13 @@
-We added an extra c10 directory so that runtime/core/portable_type/c10
+This directory contains header files from `c10` in PyTorch core that
+need to be used in ExecuTorch core. They are copied here rather than
+being found through the torch pip package to keep the core build
+hermetic for embedded use cases. The headers should be exact copies
+from PyTorch core; if they are out of sync, please send a PR!
+
+We added an extra c10 directory so that `runtime/core/portable_type/c10`
 can be the directory to put on your include path, rather than
-runtime/core/portable_type, because using runtime/core/portable_type
+`runtime/core/portable_type`, because using `runtime/core/portable_type`
 would cause all headers in that directory to be includeable with
 `#include <foo.h>`. In particular, that includes
-runtime/core/portable_type/complex.h, which would shadow the C99
-complex.h standard header.
+`runtime/core/portable_type/complex.h`, which would shadow the C99
+`complex.h` standard header.