Support flatten() for non-Shaped args

phiml/math/_magic_ops.py

@@ -587,6 +587,7 @@ def flatten(value, flat_dim: Shape = instance('flat'), flatten_batch=False, **kw
 
     Args:
         value: `phiml.math.magic.Shapable`, such as `Tensor`.
+            If a non-`phiml.math.magic.Shaped` object or one with an empty `Shape` is passed, it is returned without alteration.
         flat_dim: Dimension name and type as `Shape` object. The size is ignored.
         flatten_batch: Whether to flatten batch dimensions as well.
             If `False`, batch dimensions are kept, only onn-batch dimensions are flattened.
@@ -602,6 +603,10 @@ def flatten(value, flat_dim: Shape = instance('flat'), flatten_batch=False, **kw
         (flatⁱ=12) const 0.0
     """
     assert isinstance(flat_dim, Shape) and flat_dim.rank == 1, flat_dim
+    if not isinstance(value, Shaped):
+        return value
+    if shape(value).is_empty:
+        return value
     assert isinstance(value, Shapable) and isinstance(value, Shaped), f"value must be Shapable but got {type(value)}"
     # --- First try __flatten__ ---
     if hasattr(value, '__flatten__'):