Fix COCO Dataset Loading for Invisible Keypoints (#2035)

Update coco.py

# Fix COCO Dataset Loading for Invisible Keypoints

## Issue
When loading COCO datasets, keypoints marked as invisible (flag=0) are currently skipped and later placed randomly within the instance's bounding box. However, in COCO format, these keypoints may still have valid coordinate information that should be preserved (see toy_dataset for expected vs. current behavior).

## Changes
Modified the COCO dataset loading logic to:
- Check if invisible keypoints (flag=0) have non-zero coordinates
- If coordinates are (0,0), skip the point (existing behavior)
- If coordinates are not (0,0), create the point at those coordinates but mark it as not visible
- Maintain existing behavior for visible (flag=2) and labeled

sleap/io/format/coco.py

@@ -180,6 +180,9 @@ def read(
 
                 if flag == 0:
                     # node not labeled for this instance
+                    if (x, y) != (0, 0): 
+                        # If labeled but invisible, place the node at the coord
+                        points[node] = Point(x, y, False)
                     continue
 
                 is_visible = flag == 2