Refactor export_llama_lib.py

Summary:
Refactor the hell out of export_llama_lib.py.

All quantizer logic goes into `lib/quant_lib.py`.

All partitioner logic goes into `lib/partitioner_lib.py`.

All source transformation logic goes into `source_transformation/`.

Reviewed By: iseeyuan, cccclai

Differential Revision: D56372411

fbshipit-source-id: bfdf842980c7271aebaadfc445272fa4ca96f0d8

Author: larryliu0820

examples/models/llama2/TARGETS

@@ -27,7 +27,6 @@ runtime.python_library(
         "__init__.py",
         "fairseq2.py",
         "model.py",
-        "quantize.py",
     ],
     _is_external_target = True,
     base_module = "executorch.examples.models.llama2",
@@ -66,9 +65,12 @@ runtime.python_library(
         "builder.py",
         "export_llama.py",
         "export_llama_lib.py",
+        "lib/partitioner_lib.py",
+        "lib/quant_lib.py",
         "model.py",
-        "quant_lib.py",
-        "quantize.py",
+        "source_transformation/quantize.py",
+        "source_transformation/rope.py",
+        "source_transformation/sdpa.py",
     ],
     _is_external_target = True,
     base_module = "executorch.examples.models.llama2",