remove duplicate deepcopy

Summary:
Program is deep copied inside backend_api before being passed to preprocess:
https://www.internalfb.com/code/fbsource/fbcode/executorch/exir/backend/backend_api.py?lines=106

And then deepcopied again here.

Differential Revision: D55133796

Author: lucylq

backends/xnnpack/xnnpack_preprocess.py

@@ -86,7 +86,6 @@ def preprocess(
         edge_program: ExportedProgram,
         compile_specs: List[CompileSpec],
     ) -> PreprocessResult:
-        ep = copy.deepcopy(edge_program)
         # Need to wrap EP here because xnnpack does addmm to linear
         # transforms. This makes resulting graph not aten compliant
         # as aten.linear is not a core aten op.
@@ -97,17 +96,17 @@ def preprocess(
         # EdgeDialectVerifier, but disable it.
         # TODO (task link) to implement NullVerifier or something similar
         ep = ExportedProgram(
-            root=ep.graph_module,
-            graph=ep.graph,
-            graph_signature=ep.graph_signature,
-            state_dict=ep.state_dict,
-            range_constraints=ep.range_constraints,
-            module_call_graph=copy.deepcopy(ep.module_call_graph),
-            example_inputs=ep.example_inputs,
+            root=edge_program.graph_module,
+            graph=edge_program.graph,
+            graph_signature=edge_program.graph_signature,
+            state_dict=edge_program.state_dict,
+            range_constraints=edge_program.range_constraints,
+            module_call_graph=edge_program.module_call_graph,
+            example_inputs=edge_program.example_inputs,
             verifier=EXIREdgeDialectVerifier(
                 check_edge_ops=False, enable=False, class_only=True
             ),
-            constants=ep.constants,
+            constants=edge_program.constants,
         )
 
         passes = []