Fix problem with argmax on (0,0) arrays.

Fix matterport/Mask_RCNN#170

mrcnn/model.py

@@ -560,7 +560,11 @@ def detection_targets_graph(proposals, gt_class_ids, gt_boxes, gt_masks, config)
 
     # Assign positive ROIs to GT boxes.
     positive_overlaps = tf.gather(overlaps, positive_indices)
-    roi_gt_box_assignment = tf.argmax(positive_overlaps, axis=1)
+    roi_gt_box_assignment = tf.cond(
+        tf.greater(tf.shape(positive_overlaps)[1], 0),
+        true_fn = lambda: tf.argmax(positive_overlaps, axis=1),
+        false_fn = lambda: tf.cast(tf.constant([]),tf.int64)
+    )
     roi_gt_boxes = tf.gather(gt_boxes, roi_gt_box_assignment)
     roi_gt_class_ids = tf.gather(gt_class_ids, roi_gt_box_assignment)