Update on "[executorch][schema] Add 'EXTERNAL' to DataLocation in schema"

To indicate if a tensor is external to the PTE file or not.

Currently, we can also use the existence of 'fqn' to determine if a tensor is external or not.
I think it's better to have a specific location field as fqn may be required for cases besides external tensor storage.

Differential Revision: [D66523171](https://our.internmc.facebook.com/intern/diff/D66523171/)

[ghstack-poisoned]

Author: lucylq

exir/schema.py

@@ -55,7 +55,7 @@ class ExtraTensorInfo:
     Check program.fbs for explanations of this enum.
     """
 
-    mutable_data_segments_idx: Optional[int] = 0
+    mutable_data_segments_idx: int = 0
     fully_qualified_name: Optional[str] = None
     location: Optional[DataLocation] = None