private static TensorType outputTypeFromJson(JsonNode root, String outputName) throws IOException {
        if ( ! root.isObject() || ! root.has("outputs")) {
            return TensorType.empty;
        }
        JsonNode outputs = root.get("outputs");
        if ( ! outputs.has(outputName)) {
            return TensorType.empty;
        }
        return TensorType.fromSpec(outputs.get(outputName).asText());
    }
Defers to 'vespa-analyze-onnx-model' to determine the output type given
a set of inputs. For situations with symbolic dimension sizes that can't
easily be determined.

@author lesters
config-model/src/main/java/com/yahoo/vespa/model/ml/OnnxModelProbe.java