explanation

Signed-off-by: Masaki Kozuki <mkozuki@nvidia.com>

thunder/core/proxies.py

@@ -1937,8 +1937,19 @@ def __init__(self, *args, **kwargs):
                 self._non_tensors,
                 output=self,
             )
+            # NOTE(crcrpar): A callable being `thunder.jit`ed can call `MySubclassTensor(...)`
+            # inside of it either directly or indirectly: indirect way is to call it through
+            # a custom `torch.autograd.Function` as in
+            # https://github.com/pytorch/ao/blob/000a490/torchao/float8/float8_tensor.py#L139-L209.
+            # If it's a direct call, `trace.bound_symbols` and `trace.scopes[-1]` are identical,
+            # but not, otherwise. As [the lookasdie of `torch.autograd.Function`](
+            # https://github.com/Lightning-AI/lightning-thunder/blob/3d42c10/thunder/core/jit_ext.py#L655)
+            # puts the temporary scope to the current trace.
             current_trace = get_tracectx()
-            current_trace.scopes[-1].append(bsym)
+            if id(current_trace.bound_symbols) == id(cur_tail_scope := current_trace.scopes[-1]):
+                current_trace.add_bound_symbol(bsym)
+            else:
+                cur_tail_scope.append(bsym)
 
     def replace(self, **changes):
         r"""Return a copy of the SubclassTensorProxy object with new values for the specified fields as given to the constructor as arguments.