Fix attribute handling in autocast (#2256)

In #2091, the call to helper
was replaced with onnx ir attribute convertion. This was not properly
handling when the attribute is a subgraph and when it uses values from
the parent scope, which the ir doesn't have access to. The IR thus
raises warnings of it not being able to find those values.

This PR reverts the change.

Author: justinchuby

onnxscript/_internal/autocast.py

@@ -7,6 +7,7 @@
 
 import numpy as np
 import onnx
+import onnx.helper  # noqa: TID251
 from onnx.defs import OpSchema
 
 from onnxscript import ir, tensor
@@ -68,13 +69,9 @@ def pyvalue_to_onnx_attribute(
             name=key, type=attr_type, t=pyvalue_to_onnx_tensor(name_generator(), value)
         )
     else:
-        attr = ir.convenience.convert_attribute(
-            key,
-            value,
-            attr_type=ir.AttributeType(attr_type) if attr_type is not None else None,
-        )
-        assert isinstance(attr, ir.Attr)
-        return ir.serde.serialize_attribute(attr)
+        # When the value is a subgraph, ONNX IR will complain that some values are
+        # not found from the scope.
+        return onnx.helper.make_attribute(key, value)  # noqa: TID251
 
 
 # Utilities to convert python values into onnxscript tensors.