Remove the legacy export

Summary:
As title, try to see if we can get rid of the legacy export

Rollback Plan:

Differential Revision: D77761473

Author: cccclai

extension/llm/export/builder.py

@@ -96,7 +96,6 @@ def __init__(
         verbose: bool = False,
         metadata: Optional[dict] = None,
         dynamic_shapes: Optional[Any] = None,
-        use_legacy_export: bool = False,
         save_exported_program: bool = False,
     ):
         # Store necessary constructor arguments.
@@ -117,7 +116,6 @@ def __init__(
         self.verbose = verbose
         self.metadata = metadata
         self.dynamic_shapes = dynamic_shapes
-        self.use_legacy_export = use_legacy_export
         self.save_exported_program = save_exported_program
 
         # Note: treat this as the source of truth for the result of
@@ -228,39 +226,20 @@ def _export(self, module: Optional[torch.nn.Module] = None) -> ExportedProgram:
         # 1. torch.nn.attention.sdpa_kernel([SDPBackend.MATH]) is for bypassing the dynamo error when tracing
         # 2. torch.no_grad() is for getting rid of the dropout (not sure why training ops will show up)
         with torch.nn.attention.sdpa_kernel([SDPBackend.MATH]), torch.no_grad():
-            if self.use_legacy_export:
-                # TODO: for use cases such as qnn, which does not work with new, non-functional export IR.
-                # See issue: https://github.com/pytorch/executorch/issues/7373
-
-                with patch.object(
-                    torch._utils_internal,
-                    "export_training_ir_rollout_check",
-                    return_value=False,
-                ):
-                    # TODO: this is temporary and export_for_training doesn't work with qnn either. We need a
-                    # functional graph. See issue https://github.com/pytorch/executorch/pull/4627 for more details
-                    exported_module = torch.export.export(
-                        self.model if not module else module,
-                        self.example_inputs,
-                        self.example_kwarg_inputs,
-                        dynamic_shapes=dynamic_shape,
-                        strict=True,
-                    )
+            if module:
+                logging.info("Re-exporting with:")
             else:
-                if module:
-                    logging.info("Re-exporting with:")
-                else:
-                    logging.info("Exporting with:")
-                logging.info(f"inputs: {self.example_inputs}")
-                logging.info(f"kwargs: {self.example_kwarg_inputs}")
-                logging.info(f"dynamic shapes: {dynamic_shape}")
-                exported_module = export_for_training(
-                    self.model if not module else module,
-                    self.example_inputs,
-                    kwargs=self.example_kwarg_inputs,
-                    dynamic_shapes=dynamic_shape,
-                    strict=True,
-                )
+                logging.info("Exporting with:")
+            logging.info(f"inputs: {self.example_inputs}")
+            logging.info(f"kwargs: {self.example_kwarg_inputs}")
+            logging.info(f"dynamic shapes: {dynamic_shape}")
+            exported_module = export_for_training(
+                self.model if not module else module,
+                self.example_inputs,
+                kwargs=self.example_kwarg_inputs,
+                dynamic_shapes=dynamic_shape,
+                strict=True,
+            )
         return exported_module
 
     def export(self) -> "LLMEdgeManager":
@@ -446,13 +425,6 @@ def export_to_edge(self) -> "LLMEdgeManager":
                 self.export()
 
             override_export_behaviour = contextlib.nullcontext()
-            if self.use_legacy_export:
-                override_export_behaviour = patch.object(
-                    torch._utils_internal,
-                    "export_training_ir_rollout_check",
-                    return_value=False,
-                )
-
             with override_export_behaviour:
                 self.edge_manager = export_to_edge(
                     self.pre_autograd_graph_module,  # pyre-fixme[6]