Improved gather error message

phiml/math/_ops.py

@@ -2429,6 +2429,8 @@ def gather(values, indices: Tensor, dims: Union[DimFilter, None] = None):
     assert dims, f"No indexing dimensions for tensor {values.shape} given indices {indices.shape}"
     if dims not in values.shape:
         return expand(values, non_channel(indices))
+    assert channel(indices).rank == 1, f"indices must have a single channel dimension listing the indexed dims {dims} but got {indices.shape}."
+    assert channel(indices).volume == len(dims), f"channel dimension of indices must have size equal to the number of indexed dims {dims} but got {channel(indices)}"
     if values._is_tracer or is_sparse(values):
         if not channel(indices):
             indices = expand(indices, channel(gather=dims))