fix(decoration): cast mode.device to str for mypy return-value check

CI mypy (stricter torch stubs) catches that mode.device returns
torch.device, not str. Explicit str() cast satisfies the Optional[str]
return type annotation.

Author: johnmarktaylor91

torchlens/decoration/torch_funcs.py

@@ -109,7 +109,7 @@ def _get_active_device() -> Optional[str]:
 
         for mode in reversed(_get_current_function_mode_stack()):
             if isinstance(mode, _DeviceContext):
-                return mode.device
+                return str(mode.device)
     except (ImportError, AttributeError):
         pass
     return None