Hi, thanks for the question.
The error comes from the process_function function you used in the Engine. In this very simple example, the output of this function feeds directly into the metric as a demonstration. Note that data[0] is y_pred. It means that data[0][0] is [0.0266, 0.1719, 0.3055] and data[0][0] is [0.6886, 0.3978, 0.8176]. This can’t be correctly used in the metric.