Add ability to specify CoreML pipeline passes (#16118)

### Summary
Adds the ability to specify a set of CoreML passes as a CompileSpec,
allowing additional customization of the model compilation.

### Test plan
Converted a model and made sure it worked with a custom pipeline. Also
ensured via print statements that the passes were translated correctly.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: jgibson2

backends/apple/coreml/compiler/coreml_preprocess.py

@@ -43,6 +43,7 @@ class COMPILE_SPEC_KEYS(Enum):
     MODEL_COMPUTE_PRECISION = "model_compute_precision"
     OP_LINEAR_QUANTIZER_CONFIG = "op_linear_quantizer_config"
     ENUMERATED_SHAPES = "enumerated_shapes"
+    PASS_PIPELINE = "pass_pipeline"
 
 
 class MODEL_PATHS(Enum):
@@ -220,6 +221,33 @@ def op_linear_quantizer_config_from_compile_specs(
 
         return None
 
+    @staticmethod
+    def generate_pass_pipeline_compile_spec(pass_names: List[str]) -> CompileSpec:
+        """
+        Creates a compile spec representing the pass pipeline to be used by the CoreML backend
+        :param pass_names: the list of pass names
+        """
+        str_representation = json.dumps(pass_names)
+        byte_representation = str_representation.encode("utf-8")
+        return CompileSpec(COMPILE_SPEC_KEYS.PASS_PIPELINE.value, byte_representation)
+
+    @staticmethod
+    def pass_pipeline_from_compile_specs(
+        compile_specs: List[CompileSpec],
+    ) -> ct.PassPipeline:
+        """
+        Creates a PassPipeline from the list of compile specs, or returns the default if none are provided.
+        """
+        for compile_spec in compile_specs:
+            if compile_spec.key == COMPILE_SPEC_KEYS.PASS_PIPELINE.value:
+                pass_names_str = compile_spec.value.decode("utf-8")
+                pass_names = json.loads(pass_names_str)
+                return ct.PassPipeline(
+                    pass_names, pipeline_name="executorch_user_pipeline"
+                )
+
+        return ct.PassPipeline.DEFAULT
+
     @staticmethod
     def generate_enumerated_shapes_compile_spec(
         ep: ExportedProgram,
@@ -275,6 +303,7 @@ def generate_compile_specs(
         compute_precision: ct.precision = ct.precision.FLOAT16,
         model_type: MODEL_TYPE = MODEL_TYPE.MODEL,
         op_linear_quantizer_config: Optional[Dict] = None,
+        pass_names: Optional[List[str]] = None,
     ) -> List[CompileSpec]:
         """
         Returns the list of compile specs that's used by CoreMLBackend to lower the module.
@@ -298,6 +327,10 @@ def generate_compile_specs(
                     op_linear_quantizer_config
                 )
             )
+        if pass_names is not None:
+            compile_specs.append(
+                CoreMLBackend.generate_pass_pipeline_compile_spec(pass_names)
+            )
 
         return compile_specs
 
@@ -503,6 +536,9 @@ def preprocess(
         enumerated_shapes = CoreMLBackend.enumerated_shapes_from_compile_specs(
             compile_specs
         )
+        pass_pipeline: ct.PassPipeline = CoreMLBackend.pass_pipeline_from_compile_specs(
+            compile_specs
+        )
 
         # If using enumerated shapes, we need to pass the inputs to CoreML's convert() function
         # explicitly
@@ -530,7 +566,7 @@ def preprocess(
             model=edge_program,
             source="pytorch",
             convert_to="mlprogram",
-            pass_pipeline=ct.PassPipeline.DEFAULT,
+            pass_pipeline=pass_pipeline,
             skip_model_load=skip_model_load,
             compute_precision=model_compute_precision,
             minimum_deployment_target=minimum_deployment_target,