[fx] Fix GraphModule.print_readable() (#88730)

Summary: `__nested_code()` seems removed.

Test Plan: CI

Differential Revision: D41149662

Pull Request resolved: #88730
Approved by: https://github.com/SherlockNoMad

torch/fx/graph_module.py

@@ -725,7 +725,7 @@ def print_readable(self, print_output=True):
         submodule_code_list = [""]
         for submodule in self.children():
             if isinstance(submodule, GraphModule):
-                submodule_code_list.append(submodule.__nested_code())
+                submodule_code_list.append(submodule.print_readable(print_output=False))
         submodule_code = "\n".join(submodule_code_list)
         submodule_code = _addindent(submodule_code, 4)