The flatten loop in VBenchPromptDataset strips exactly one level of
upstream's {dim_name: ...} wrapper. For most VBench dims this leaves
a flat scalar dict ({"color": "red"}, {"object": "person"}). For
spatial_relationship, upstream double-wraps, so one level of unwrap
leaves {"spatial_relationship": {object_a, object_b, relationship}} —
which is exactly what SpatialRelationshipMetric reads.
That last case looks accidental; it isn't. Documenting all four shapes
inline so a future reader doesn't "simplify" the wrapping away and
silently break spatial_relationship scoring.
Empirically verified output for all four aux dims matches the
documented shapes.
shaoxiongduan
b07cc3f9d4