Update base for 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

exir/emit/_emitter.py

@@ -77,6 +77,7 @@
     ScalarType,
     String,
     Tensor,
+    TensorDataLocation,
     TensorList,
     TensorShapeDynamism,
 )
@@ -372,8 +373,8 @@ def _save_new_const_tensor(
         spec: TensorSpec,
         buffer_data: bytes,
         hashed: str,
-        allocation_info: Optional[AllocationDetails],
-        constant_tag: str,
+        allocation_info: Optional[AllocationDetails] = None,
+        constant_tag: Optional[str] = None,
     ) -> int:
         """Saves a new constant tensor to the constant buffer and returns the buffer idx"""
 
@@ -395,7 +396,7 @@ def _save_new_const_tensor(
             if (
                 spec.extra_tensor_info is not None
                 and spec.extra_tensor_info.fully_qualified_name is not None
-                and spec.extra_tensor_info.location == DataLocation.EXTERNAL
+                and spec.extra_tensor_info.location == TensorDataLocation.EXTERNAL
             ):
                 assert (
                     constant_tag is not None
@@ -460,7 +461,7 @@ def _tensor_spec_to_evalue(
                 )
             elif (
                 spec.extra_tensor_info is not None
-                and spec.extra_tensor_info.location == DataLocation.EXTERNAL
+                and spec.extra_tensor_info.location == TensorDataLocation.EXTERNAL
             ):
                 buffer_idx = self.program_state.external_constant_hash.get(hashed, -1)
             else:
@@ -1614,7 +1615,7 @@ def placeholder(
                 ), "constant tagged tensors require a fully qualified name"
                 if spec.extra_tensor_info is None:
                     spec.extra_tensor_info = ExtraTensorInfo(
-                        fully_qualified_name=fqn, location=DataLocation.EXTERNAL
+                        fully_qualified_name=fqn, location=TensorDataLocation.EXTERNAL
                     )
                 else:
                     spec.extra_tensor_info.fully_qualified_name = fqn