Fix bug in dynamic shapes multiply (pytorch#90336)

Pull Request resolved: pytorch#90336
Approved by: https://github.com/ezyang

Author: tugsbayasgalan

test/dynamo/test_repros.py

@@ -2127,6 +2127,18 @@ def compiled_fn(x):
             for buffer_ref, buffer_test in zip(m_ref.buffers(), m_test.buffers()):
                 self.assertTrue(same(buffer_ref, buffer_test))
 
+    @patch.object(torch._dynamo.config, "dynamic_shapes", True)
+    def test_dynamic_shapes_right_side(self):
+        def f(x):
+            return torch.ones(5 * x.shape[0])
+
+        inp = torch.randn(6, 5)
+
+        gm, _ = torch._dynamo.export(
+            f, torch.randn(4, 5), aten_graph=True, tracing_mode="symbolic"
+        )
+        self.assertEqual(gm(inp).shape, f(inp).shape)
+
 
 if __name__ == "__main__":
     from torch._dynamo.test_case import run_tests

torch/_dynamo/variables/builtin.py

@@ -576,6 +576,11 @@ def call_mul(self, tx, a, b):
             return b.__class__(
                 items=b.items * a.as_python_constant(), mutable_local=MutableLocal()
             ).add_options(self, a, b)
+        # TODO this doesn't generalize in other builtin operators.
+        elif isinstance(a, variables.ConstantVariable) and isinstance(
+            b, DynamicShapeVariable
+        ):
+            return b.call_method(tx, "__rmul__", [a], {})
         else:
             return a.call_method(tx, "__mul__", [b], {})