Fixed coco_eval bug where seg length check could be wrong (#1376) (#1384)

* Fixed coco_eval bug where seg length check could be wrong (#1376)

* Added check for segm counts field to check if the decode method is present

* Changed classes length check to more general check

* Fixed coco_utils.py to be more robust condition for mask lasbelling

* Added seg length check to ensure two objects exist

* Added type check over len check

* Check for only tuple