Update on "Save foundation weights separately"

This diff:
1. Introduces SerializationConfig to llm_config. Currently, this allows user to save the foundation weights in a separate file; majorly useful for lora case.
2. Adds a pass to tag foundation (non-lora) weights. This is at the top-level (export_llama_lib). The tags are preserved through run_decomps/other passes, and do not affect functionality.
3. Tags are read when placing constants into the named_data_store.
4. Tagged weights are serialized to a separate file.


Notes
1. Adding tags to node.meta['custom']['blah'] means that they will not be discarded by run_decompositions
2. Adding tags to the lifted model (ep.graph_module) requires the EP to check is_param_node for xnnpack constants. Instead, add tags to the unlifted model (ep.module()), so we do not need to go through a re-export to get the EP.
3. Not an issue for this diff as llama doesn't have any higher order ops. Adding tags to models with higher-order ops is problematic due to nested submodules.

Differential Revision: [D79181064](https://our.internmc.facebook.com/intern/diff/D79181064/)

[ghstack-poisoned]

Author: lucylq

.ci/scripts/test_llama_lora.sh

@@ -84,8 +84,8 @@ RESULT=$(cat result.txt)
 if [[ "${RESULT}" == "${EXPECTED_PREFIX}"* ]]; then
   echo "Expected result prefix: ${EXPECTED_PREFIX}"
   echo "Actual result: ${RESULT}"
+  # Do not clean up files if test passes, as they're re-used in the next test.
   echo "Success"
-  cleanup_files
 else
   echo "Expected result prefix: ${EXPECTED_PREFIX}"
   echo "Actual result: ${RESULT}"

backends/xnnpack/operators/node_visitor.py

@@ -622,7 +622,9 @@ def get_serialized_buffer_index(
         )
 
         custom_meta = tensor.meta.get("custom", None)
-        external_tag = custom_meta.get("delegate_constant_tag", None) if custom_meta else None
+        external_tag = (
+            custom_meta.get("delegate_constant_tag", None) if custom_meta else None
+        )
         if external_tag is not None:
             external_tag = custom_meta.get("delegate_constant_tag", None)
             logging.info(