Fix how tcn test considers total num classes automatically

[Purg]

Some detection inputs have "missing" categories due to their being skipped during detection inference. This creates a discrepancy between quantity of listed categories vs category IDs that can be listed. Instead consider the max category ID + 1, on the assumption that category IDs start at 0, which is a strong assumption due to considering category IDs as the index of prediction outputs to which that category applies.