[fx] improve args mutation error (#51175)

Summary:
Pull Request resolved: #51175

gives a suggestion about how to deal with immutable args/kwargs list

Test Plan: Imported from OSS

Reviewed By: jamesr66a

Differential Revision: D26093478

Pulled By: zdevito

fbshipit-source-id: 832631c125561c3b343539e887c047f185060252

test/test_fx.py

@@ -1438,6 +1438,12 @@ def forward(self, x):
             i += 1
         self.assertEqual(i, 3)
 
+    def test_no_mutation(self):
+        from torch.fx.immutable_collections import immutable_list
+        x = immutable_list([3, 4])
+        with self.assertRaisesRegex(NotImplementedError, "new_args"):
+            x[0] = 4
+
 
 def run_getitem_target():
     from torch.fx.symbolic_trace import _wrapped_methods_to_patch

torch/fx/immutable_collections.py

@@ -1,5 +1,13 @@
+
+_help_mutation = """\
+If you are attempting to modify the kwargs or args of a torch.fx.Node object,
+instead create a new copy of it and assign the copy to the node:
+    new_args = ... # copy and mutate args
+    node.args = new_args
+"""
+
 def _no_mutation(self, *args, **kwargs):
-    raise NotImplementedError(f"'{type(self).__name__}' object does not support mutation")
+    raise NotImplementedError(f"'{type(self).__name__}' object does not support mutation. {_help_mutation}")
 
 def _create_immutable_container(base, mutable_functions):
     container = type('immutable_' + base.__name__, (base,), {})