[dynamo] Graph break on on user-defined class in compiled region (#161670)

Currently, user-defined classes inside of a compiled frame will cause the whole
frame to be skipped by dynamo.  This change defers the Unsupported exception
until the __build_class__ builtin is actually called, which allows a graph break
to be inserted.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/161670
Approved by: https://github.com/williamwen42, https://github.com/guilhermeleobas

test/dynamo/test_error_messages.py

@@ -726,14 +726,14 @@ Call to `torch._dynamo.graph_break()`
             Unsupported,
             lambda: torch.compile(fn, backend="eager", fullgraph=True)(),
             """\
-LOAD_BUILD_CLASS bytecode not supported
-  Explanation: Dynamo does not support tracing classes that are defined in the compiled region.
-  Hint: Move the class definition out of the compiled region.
-  Hint: It may be possible to write Dynamo tracing rules for this code. Please report an issue to PyTorch if you encounter this graph break often and it is causing performance issues.
+Attempted to call function marked as skipped
+  Explanation: Dynamo does not know how to trace the builtin `builtins.__build_class__.` This function is either a Python builtin (e.g. _warnings.warn) or a third-party C/C++ Python extension (perhaps created with pybind).
+  Hint: If it is a Python builtin, please file an issue on GitHub so the PyTorch team can add support for it and see the next case for a workaround.
+  Hint: If it is a third-party C/C++ Python extension, please either wrap it into a PyTorch-understood custom operator (see https://pytorch.org/tutorials/advanced/custom_ops_landing_page.html for more details) or, if it is traceable, use `torch.compiler.allow_in_graph`.
 
-  Developer debug context:
+  Developer debug context: module: builtins, qualname: __build_class__, skip reason: <missing reason>
 
- For more details about this graph break, please visit: https://meta-pytorch.github.io/compile-graph-break-site/gb/gb0075.html
+ For more details about this graph break, please visit: https://meta-pytorch.github.io/compile-graph-break-site/gb/gb0007.html
 
 from user code:
    File "test_error_messages.py", line N, in fn

test/dynamo/test_misc.py

@@ -12684,6 +12684,22 @@ fn
         self.assertRaises(Unsupported, f, [])
         self.assertRaises(Unsupported, f, "1 + j")
 
+    def test_compiled_class_graph_break(self):
+        counter = CompileCounter()
+
+        @torch.compile(backend=counter, fullgraph=False)
+        def f(x):
+            x += 1
+
+            class C:
+                pass
+
+            return x.sin()
+
+        x = torch.randn(3)
+        f(x)
+        self.assertEqual(counter.frame_count, 2)
+
 
 class MiscTestsPyTree(torch._inductor.test_case.TestCase):
     @parametrize_pytree_module