remove unused import from op_attention.py

Author: Sanggyu Lee

tico/serialize/operators/onert/op_attention.py

@@ -20,7 +20,7 @@
 import torch
 from circle_schema import circle
 
-from tico.serialize.circle_graph import CircleSubgraph, extract_shape
+from tico.serialize.circle_graph import CircleSubgraph
 from tico.serialize.operators.hashable_opcode import OpCode
 from tico.serialize.operators.node_visitor import NodeVisitor, register_node_visitor
 from tico.serialize.operators.utils import create_builtin_operator, get_op_index