Update on "Use unlifted export pass to tag delegated constants"

Use the unlifted pass to tag constants for delegates.

Implications:
- Tagging must happen on the unlifted ep.module(), before going into to_edge_transform_and_lower/to_edge.

Why?
- The unlifted graph contains constants in getattr nodes, which is a convenient way to isolate constants.
- After going into to_edge_transform_and_lower/to_edge, transforms happen on the graph_module, which is lifted.
- The lifted graph requires the ep graph signature to differentiate constants via the `is_param` function. However, in to_edge.transform, we do not have access to the ep. Baking the ep as an argument via partial function doesn't work, as the ep from earlier may be outdated. This means we are comparing an older ep to a newer graph_module, which may not have corresponding graph signatures etc.

Differential Revision: [D79736684](https://our.internmc.facebook.com/intern/diff/D79736684/)

[ghstack-poisoned]

Author: lucylq

examples/models/llama/export_llama_lib.py

@@ -1088,7 +1088,10 @@ def _export_llama(llm_config: LlmConfig) -> LLMEdgeManager:  # noqa: C901
             from executorch.exir.passes.external_constants_pass import (
                 delegate_external_constants_pass_unlifted,
             )
-            assert builder_exported.pre_autograd_graph_module is not None, "pre_autograd_graph_module shouldn't be None here"
+
+            assert (
+                builder_exported.pre_autograd_graph_module is not None
+            ), "pre_autograd_graph_module shouldn't be None here"
             delegate_external_constants_pass_unlifted(
                 module=builder_exported.pre_autograd_graph_module,
                 gen_tag_fn=gen_tag_fn,

test/models/export_delegated_program.py

@@ -171,6 +171,7 @@ def forward(self, *args, **kwargs):
         from executorch.backends.xnnpack.partition.xnnpack_partitioner import (
             XnnpackPartitioner,
         )
+
         if external_constants:
             tagged_module = exported_program.module()
             delegate_external_constants_pass_unlifted(