corrected bug for not counts gts

[iocalangiu]

in detection_metrics.py ground truths are not counted if the update()function ends early with no predictions.

I moved counting ground truths:

# Update ground truth counts
for g_label in gt_labels:
      self.gt_counts[int(g_label)] += 1

prior to all early returns.

[dpascualhe]

Looking good! Thanks for your contribution 😄