fix tanh typo test=develop (#16049)

paddle/fluid/operators/ngraph/ops/activation_op.h

@@ -55,4 +55,4 @@ void BuildTanhGradNode(
 }  // namespace paddle
 
 REGISTER_NG_OP(relu_grad, BuildReluGradNode);
-REGISTER_NG_OP(than_grad, BuildTanhGradNode);
+REGISTER_NG_OP(tanh_grad, BuildTanhGradNode);