DataTypes should be URIRef (#714)

We should always convert DataTypes from string format to URIRef.

Currently, we do it only in some cases, it should be done consistently.

python/mlcroissant/mlcroissant/_src/core/data_types.py

@@ -34,7 +34,10 @@ def check_expected_type(issues: Issues, jsonld: Json, expected_type: str):
 def data_types_from_jsonld(ctx: Context, data_types: Json):
     """Extracts DataType from its JSON-LD."""
     if isinstance(data_types, dict):
-        return data_types.get("@id")
+        data_type = data_types.get("@id")
+        if isinstance(data_type, str):
+            data_type = term.URIRef(data_type)
+        return data_type
     elif isinstance(data_types, (str, term.URIRef)):
         return term.URIRef(data_types)
     elif isinstance(data_types, list):