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

@@ -44,7 +44,7 @@ class TensorShapeDynamism(IntEnum):
 
 
 class TensorDataLocation(IntEnum):
-    CONSTANT_SEGMENT = 0
+    SEGMENT = 0
     EXTERNAL = 1
 
 
@@ -56,7 +56,7 @@ class ExtraTensorInfo:
 
     mutable_data_segments_idx: int = 0
     fully_qualified_name: Optional[str] = None
-    location: TensorDataLocation = TensorDataLocation.CONSTANT_SEGMENT
+    location: TensorDataLocation = TensorDataLocation.SEGMENT
 
 
 @dataclass

schema/program.fbs

@@ -67,16 +67,17 @@ table ExtraTensorInfo {
   // [Optional] Specifies the SubsegmentOffsets in
   //  program.mutable_data_segments that specifies where the data is located in.
   //  If not present and the data is located in a segment, then the data is in
-  //  the first index.
+  //  index zero.
   mutable_data_segments_idx: uint64;
 
   // [Optional] The unique name of the tensor. e.g. 'mod.linear.weight'
   fully_qualified_name: string;
 
-  // [Optional] Specifies where the tensor is stored; either in the constant segment
-  // (default case) or an external location. If a constant tensor is stored externally,
-  // data_buffer_idx is not relevant; use extra_tensor_info.fully_qualified_name to
-  // match up the external tensor.
+  // [Optional] Specifies where the tensor's data is stored.
+  // - SEGMENT (default): Data is stored in a segment.
+  // - EXTERNAL: Data is stored outside of the PTE file. fully_qualified_name
+  //   must be non-empty, and is used as a key to find the tensor's external
+  //   data. Tensor.data_buffer_idx is ignored.
   location: TensorDataLocation;
 }