[inductor][subgraph] Plumbing to get ShapeAsConstantBuffer from subgraph to main graph output (#147559)

I am unable to create a test case that fails without the next PR. The idea is to have a symint which is returned by the inner subgraph and then returned by the forward graph after partitioning.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/147559
Approved by: https://github.com/eellison

torch/_inductor/graph.py

@@ -1269,6 +1269,7 @@ class GraphLowering(torch.fx.Interpreter):
                     sympy.logic.boolalg.Boolean,
                     int,
                     ir.EffectfulKernel,
+                    ir.ShapeAsConstantBuffer,
                 ),
             )
             for x in result

torch/_inductor/ir.py

@@ -208,6 +208,7 @@ def validate_ir(node_or_nodes: Optional[_NodeOrNodes]) -> None:
                     Expr,
                     int,
                     EffectfulKernel,
+                    ShapeAsConstantBuffer,
                 ),
             ), (
                 f"Found {type(nodes)}, which is not a supported top level IR node. See [Note: Inductor IR]"
@@ -5166,7 +5167,7 @@ class ExternKernel(InputsKernel):
             # TODO(jansel): impose layout preference on realized buffer
             x.realize()
             return x
-        if isinstance(x, (NonTensorObj)):
+        if isinstance(x, (NonTensorObj, ShapeAsConstantBuffer)):
             return x
         return cls.copy_input(x)
 
@@ -7041,6 +7042,8 @@ class MutableBox(IRNode):
 class TensorBox(MutableBox):
     @staticmethod
     def create(data):  # type: ignore[no-untyped-def]
+        if isinstance(data, ShapeAsConstantBuffer):
+            return data
         return TensorBox(StorageBox(data))