added type hints and fixed potential bug

Signed-off-by: Lukas Folle <lukas.folle@snke.com>

Author: lukas-folle-snkeos

monai/transforms/utility/array.py

@@ -1973,10 +1973,17 @@ class FlattenSequence(Transform, ReduceTrait):
     def __init__(self):
         super().__init__()
 
-    def __call__(self, data):
-        if isinstance(data, (list, tuple)):
-            if len(data) == 0:
-                return data
-            if isinstance(data[0], (list, tuple)):
-                return [item for sublist in data for item in sublist]
-        return data
+def __call__(self, data: list | tuple | Any) -> list | tuple | Any:
+    """
+    Flatten a nested sequence by one level.
+    Args:
+        data: Input data, can be a nested sequence.
+    Returns:
+        Flattened list if input is a nested sequence, otherwise returns data unchanged.
+    """
+    if isinstance(data, (list, tuple)):
+        if len(data) == 0:
+            return data
+        if all(isinstance(item, (list, tuple)) for item in data):
+            return [item for sublist in data for item in sublist]
+    return data