[Pattern Matcher] improve error msg (pytorch#161423)

Updates pattern matcher error message

Pull Request resolved: pytorch#161423
Approved by: https://github.com/mengluy0125, https://github.com/masnesral

Author: mlazos

torch/_inductor/pattern_matcher.py

@@ -1306,7 +1306,9 @@ def replace(
                 for user in old_uses:
                     idx = maybe_getitem(user)
                     if idx is None:
-                        raise AssertionError("can't handle")
+                        raise AssertionError(
+                            "Deleted index from getitem, did you erase the index and not properly replace it?"
+                        )
                     replace(user, new[idx])
                 graph.erase_node(old)