Prohibit nested backends

Summary: pytorch#15528 initially wanted to subclass a backend.. It was currently already guarded by https://github.com/pytorch/executorch/blob/main/exir/backend/backend_api.py#L111-L112 meaning that subclass will not show up.  However it's not super obvious so we want to guard by disallowing subclass at all

Differential Revision: D87105211

Author: cccclai

exir/backend/backend_details.py

@@ -57,6 +57,22 @@ class BackendDetails(ABC):
 
     """
 
+    def __init_subclass__(cls, **kwargs):
+        super().__init_subclass__(**kwargs)
+
+        # Allow direct subclasses of BackendDetails
+        if cls.__bases__ == (BackendDetails,):
+            return
+
+        # Forbid subclasses whose ANY parent is already a child of BackendDetails
+        for base in cls.__bases__:
+            if issubclass(base, BackendDetails) and base is not BackendDetails:
+                raise TypeError(
+                    f"ExecuTorch delegate doesn't support nested backend, '{base.__name__}' "
+                    " should be a final backend implementation and should not be subclassed "
+                    f"(attempted by '{cls.__name__}')."
+                )
+
     @staticmethod
     # all backends need to implement this method
     @enforcedmethod

exir/backend/test/test_backends.py

@@ -12,6 +12,7 @@
 import torch
 from executorch.exir import to_edge
 from executorch.exir.backend.backend_api import LoweredBackendModule, to_backend
+from executorch.exir.backend.backend_details import BackendDetails
 from executorch.exir.backend.canonical_partitioners.all_node_partitioner import (
     AllNodePartitioner,
 )
@@ -1444,3 +1445,18 @@ def inputs(self):
             self.assertTrue(
                 torch.allclose(model_outputs[0], ref_output, atol=1e-03, rtol=1e-03)
             )
+
+    def test_prohibited_nested_backends(self):
+        class MyBackend(BackendDetails):
+            @staticmethod
+            def preprocess(edge_program, compile_specs):
+                return None
+
+        with self.assertRaises(TypeError) as ctx:
+            class MyOtherBackend(MyBackend):
+                pass
+
+        self.assertIn(
+            "'MyBackend'  should be a final backend implementation and should not be subclassed (attempted by 'MyOtherBackend')", 
+            str(ctx.exception)
+        )