feat: Add chain_sum() factory function to create ChainSum dataset instances

reasoning_gym/arithmetic/basic_arithmetic.py

@@ -142,6 +142,41 @@ class ArithmeticDataset:
         expression = " ".join(expression_parts)
         return expression, result
 
+
+def chain_sum(
+    min_terms: int = 2,
+    max_terms: int = 6,
+    min_digits: int = 1,
+    max_digits: int = 4,
+    allow_negation: bool = False,
+    seed: Optional[int] = None,
+    size: int = 500,
+) -> ChainSum:
+    """Create a ChainSum dataset with the given configuration.
+    
+    Args:
+        min_terms: Minimum number of terms in expressions
+        max_terms: Maximum number of terms in expressions
+        min_digits: Minimum number of digits in numbers
+        max_digits: Maximum number of digits in numbers
+        allow_negation: Whether to allow negative numbers
+        seed: Random seed for reproducibility
+        size: Virtual size of the dataset
+        
+    Returns:
+        ChainSum: Configured dataset instance
+    """
+    config = ChainSumConfig(
+        min_terms=min_terms,
+        max_terms=max_terms,
+        min_digits=min_digits,
+        max_digits=max_digits,
+        allow_negation=allow_negation,
+        seed=seed,
+        size=size,
+    )
+    return ChainSum(config)
+
     def __iter__(self):
         """Make the dataset iterable"""
         self._current_idx = 0