[BE][export] Change custom_op registeration style (#145315)

Summary:
`test_unbacked_bindings_for_divisible_u_symint` has been flaky for a while due to

```
Tried to register an operator (mylib::foo(Tensor a, Tensor b) -> Tensor) with the same name and overload name multiple times.
```

It is likely due to when all variants of this test are being run (non-strict, retrace, serdes) simultaneously. In later tests, the operator has already been registered.

In this diff, we change registration style.

Test Plan:
```
buck2 test mode/dev-nosan caffe2/test:test_export -- -r test_unbacked_bindings_for_divisible_u_symint
```

Differential Revision: D68465258

Pull Request resolved: https://github.com/pytorch/pytorch/pull/145315
Approved by: https://github.com/zou3519

test/export/test_export.py

@@ -3714,43 +3714,35 @@ def forward(self, p_linear_weight, p_linear_bias, b_buffer, x):
     @testing.expectedFailureCppSerDes  # no unbacked bindings after deserialization?
     @testing.expectedFailureSerDerNonStrict
     def test_unbacked_bindings_for_divisible_u_symint(self):
-        with torch.library._scoped_library("mylib", "FRAGMENT") as lib:
-            torch.library.define(
-                "mylib::foo",
-                "(Tensor a, Tensor b) -> (Tensor)",
-                tags=torch.Tag.pt2_compliant_tag,
-                lib=lib,
-            )
+        class M(torch.nn.Module):
+            def forward(self, a, b):
+                return torch.ops.mylib.foo_unbacked(a, b)
 
-            class M(torch.nn.Module):
-                def forward(self, a, b):
-                    return torch.ops.mylib.foo(a, b)
+        @torch.library.custom_op("mylib::foo_unbacked", mutates_args={})
+        def foo_unbacked(a: torch.Tensor, b: torch.Tensor) -> torch.Tensor:
+            return a[b.item()]
 
-            @torch.library.impl("mylib::foo", "cpu", lib=lib)
-            def foo_impl(a, b):
-                return a[b.item()]
+        @foo_unbacked.register_fake
+        def foo_unbacked_fake_impl(a, b):
+            ctx = torch.library.get_ctx()
+            u = ctx.new_dynamic_size(min=0, max=len(a) // 10) * 10
+            return torch.empty(u, a.shape[1], dtype=a.dtype)
 
-            @torch.library.register_fake("mylib::foo", lib=lib)
-            def foo_fake_impl(a, b):
-                ctx = torch.library.get_ctx()
-                u = ctx.new_dynamic_size(min=0, max=len(a) // 10) * 10
-                return torch.empty(u, a.shape[1], dtype=a.dtype)
-
-            ep = export(
-                M(),
-                (torch.randn(100, 4), torch.tensor(10)),
-            )
-            foo = [node for node in ep.graph.nodes if node.name == "foo"][0]
-            unbacked_bindings = foo.meta["unbacked_bindings"]
-            self.assertEqual(len(unbacked_bindings), 1)  # check binding is {u: path}
-            u = next(iter(unbacked_bindings.keys()))
-            self.assertEqual(
-                type(u).__name__, "Symbol"
-            )  # check binding is symbol, not expr
-            path = unbacked_bindings[u]
-            self.assertEqual(len(path), 3)  # check path is [size, 0, DivideByKey(10)]
-            self.assertEqual(type(path[2]).__name__, "DivideByKey")
-            self.assertEqual(path[2].divisor, 10)
+        ep = export(
+            M(),
+            (torch.randn(100, 4), torch.tensor(10)),
+        )
+        foo = [node for node in ep.graph.nodes if node.name == "foo_unbacked"][0]
+        unbacked_bindings = foo.meta["unbacked_bindings"]
+        self.assertEqual(len(unbacked_bindings), 1)  # check binding is {u: path}
+        u = next(iter(unbacked_bindings.keys()))
+        self.assertEqual(
+            type(u).__name__, "Symbol"
+        )  # check binding is symbol, not expr
+        path = unbacked_bindings[u]
+        self.assertEqual(len(path), 3)  # check path is [size, 0, DivideByKey(10)]
+        self.assertEqual(type(path[2]).__name__, "DivideByKey")
+        self.assertEqual(path[2].divisor, 10)
 
     def test_torch_check_eq_commutativity(self):
         class M1(torch.nn.Module):