[torch.compile] Stop doing unnecessary FakeTensorProp in PiecewiseCompileInterpreter (#34093)

Signed-off-by: Richard Zou <zou3519@gmail.com>

tests/compile/fullgraph/test_simple.py

@@ -27,10 +27,29 @@ from ...utils import create_new_process_for_each_test
 from ..silly_attention import get_global_counter, reset_global_counter
 
 
+# Custom op that returns an unbacked symint during graph capture
+@torch.library.custom_op("mylib::foo", mutates_args=())
+def foo(x: torch.Tensor) -> int:
+    return 3
+
+
+@foo.register_fake
+def _(x):
+    return torch.library.get_ctx().new_dynamic_size()
+
+
 @support_torch_compile
 class SillyModel(nn.Module):
-    def __init__(self, *, vllm_config: VllmConfig, prefix: str = "", **kwargs) -> None:
+    def __init__(
+        self,
+        *,
+        vllm_config: VllmConfig,
+        prefix: str = "",
+        intermediate_unbacked=False,
+        **kwargs,
+    ) -> None:
         super().__init__()
+        self.intermediate_unbacked = intermediate_unbacked
 
     def forward(self, x: torch.Tensor) -> torch.Tensor:
         """
@@ -44,6 +63,13 @@ class SillyModel(nn.Module):
         torch.ops.silly.attention(x, x, x, out)
         x = out
         x = x - 2
+
+        if self.intermediate_unbacked:
+            # Test for unbacked symints: the following is a fancy way to multiply by 1
+            u0 = foo(x)
+            ones = x.new_ones(x.shape[0], u0).sum(-1) / 3
+            x = x * ones
+
         x = x - 1
         out = torch.empty_like(x)
         torch.ops.silly.attention(x, x, x, out)
@@ -52,6 +78,7 @@ class SillyModel(nn.Module):
         return x
 
 
+@torch._dynamo.config.patch(capture_dynamic_output_shape_ops=True)
 def _run_simple_model(
     splitting_ops,
     use_inductor_graph_partition,
@@ -60,6 +87,8 @@ def _run_simple_model(
     expected_num_piecewise_capturable_graphs_seen,
     expected_num_backend_compilations,
     expected_num_cudagraph_captured,
+    *,
+    intermediate_unbacked=False,
 ):
     vllm_config = VllmConfig(
         compilation_config=CompilationConfig(
@@ -72,7 +101,11 @@ def _run_simple_model(
         )
     )
     with set_current_vllm_config(vllm_config):
-        model = SillyModel(vllm_config=vllm_config, prefix="")
+        model = SillyModel(
+            vllm_config=vllm_config,
+            prefix="",
+            intermediate_unbacked=intermediate_unbacked,
+        )
 
     inputs = torch.randn(100).cuda()
 
@@ -125,9 +158,10 @@ def _run_simple_model(
 
 
 @pytest.mark.parametrize("backend", ["inductor", "eager"])
+@pytest.mark.parametrize("intermediate_unbacked", [True, False])
 @torch.inference_mode()
 @create_new_process_for_each_test("spawn")
-def test_simple_piecewise_compile(backend):
+def test_simple_piecewise_compile(backend, intermediate_unbacked):
     _run_simple_model(
         splitting_ops=["silly::attention"],
         use_inductor_graph_partition=False,
@@ -140,6 +174,7 @@ def test_simple_piecewise_compile(backend):
         expected_num_backend_compilations=3,
         # num_cudagraph_sizes * num_piecewise_capturable_graphs_seen
         expected_num_cudagraph_captured=6,
+        intermediate_unbacked=intermediate_unbacked,
     )