Update on "[executorch][passes] Add config and pass to tag constants for external file"

- Add config 'external_constants' to ExecutorchBackendConfig.
- When set to True, run the 'external_constants_pass'
- This tags all constants as external, and moves them into a separate buffer to be serialized outside of the PTE file.

Note: users can write their own passes to tag weights to specific files / multiple files.

TODO: write example pass and test for the case where we have two constant files.

Differential Revision: [D66560903](https://our.internmc.facebook.com/intern/diff/D66560903/)

[ghstack-poisoned]

Author: lucylq

examples/portable/scripts/export.py

@@ -6,6 +6,8 @@
 
 # Example script for exporting simple models to flatbuffer
 
+# pyre-unsafe
+
 import argparse
 import logging