Keep LiftedConstant in .pte (pytorch#9202)

Summary:
See pytorch#8809 for context

LiftedConstants should not be moved to external file (data), as they are
closer semantically to code

Differential Revision: D71064053

Author: Lucaskabela

exir/emit/test/test_emit.py

@@ -1534,26 +1534,49 @@ def forward(self, x):
         self.assertEqual(len(program.constant_buffer[1].storage), 8)
 
     def test_emit_lifted_tensor_constant(self) -> None:
-        class LiftedConstants(nn.Module):
+        class LiftedTensorConstants(nn.Module):
             def __init__(self):
                 super().__init__()
 
             def forward(self, x):
                 x = x * torch.tensor([[4, 3], [1, 2], [5, 6]], dtype=torch.float)
                 return x
 
-        model = LiftedConstants()
+        model = LiftedTensorConstants()
+        # Specify that we want to move non-lifted constants to external file
+        et_cfg = ExecutorchBackendConfig(external_constants=True)
+        program = to_edge(
+            export(model, (torch.ones(3, 2),), strict=True)
+        ).to_executorch(et_cfg)
+        program = program._emitter_output.program
+        exec_plan = program.execution_plan[0]
+        # There should only be 1 input to this model.
+        self.assertEqual(len(exec_plan.inputs), 1)
+        self.assertEqual(len(program.constant_buffer), 2)
+        self.assertEqual(len(program.constant_buffer[1].storage), 24)
 
+    def test_emit_lifted_constant(self) -> None:
+        class LiftedConstants(nn.Module):
+            def __init__(self):
+                super().__init__()
+
+            def forward(self, x):
+                x = x + 1
+                return x
+
+        model = LiftedConstants()
+        # Specify that we want to move non-lifted constants to external file
+        et_cfg = ExecutorchBackendConfig(external_constants=True)
         program = to_edge(
             export(model, (torch.ones(3, 2),), strict=True)
-        ).to_executorch()
+        ).to_executorch(et_cfg)
 
         program = program._emitter_output.program
         exec_plan = program.execution_plan[0]
         # There should only be 1 input to this model.
         self.assertEqual(len(exec_plan.inputs), 1)
         self.assertEqual(len(program.constant_buffer), 2)
-        self.assertEqual(len(program.constant_buffer[1].storage), 24)
+        self.assertEqual(len(program.constant_buffer[1].storage), 8)
 
     def test_mutable_buffers(self) -> None:
         def count_copies(gm: torch.fx.GraphModule) -> int:

exir/passes/external_constants_pass.py

@@ -17,15 +17,17 @@ def external_constants_pass(
     gm: GraphModule,
 ) -> PassResult:
     """
-    Move all constants to external file.
+    Move all non-lifted constants to external file.
+    NOTE: Lifted constants are not moved as they are closer
+    to code than data.
     """
     mutated = False
     for module in gm.modules():
         if not isinstance(module, torch.fx.GraphModule):
             continue
 
         for node in module.graph.nodes:
-            if node.op == "placeholder":
+            if (node.op == "placeholder") and ("_lifted_tensor" not in node.name):
                 spec = node.meta.get("spec")
                 if isinstance(spec, TensorSpec) and spec.const:
                     node.meta["constant_tag"] = "_default_external_constant"