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

@@ -43,20 +43,21 @@ class TensorShapeDynamism(IntEnum):
     DYNAMIC_UNBOUND = 2
 
 
+class DataLocation(IntEnum):
+    INLINE = 0
+    SEGMENT = 1
+    EXTERNAL = 2
+
+
 @dataclass
 class ExtraTensorInfo:
     """
     Check program.fbs for explanations of this enum.
     """
 
-    mutable_data_segments_idx: Optional[int] = None
+    mutable_data_segments_idx: Optional[int] = 0
     fully_qualified_name: Optional[str] = None
-
-
-class DataLocation(IntEnum):
-    INLINE = 0
-    SEGMENT = 1
-    EXTERNAL = 2
+    location: Optional[DataLocation] = None
 
 
 @dataclass