Fix _linear_solve_forward()

phiml/math/_optimize.py

@@ -714,7 +714,7 @@ def _linear_solve_forward(y: Tensor,
         matrix_offset = reshaped_tensor(approx_matrix_vals, [batch_dims])
         matrix_offset = math.where(solve.rank_deficiency > 0, matrix_offset, 0).native([batch_dims])
     method = solve.method
-    if not callable(native_lin_op) and is_sparse(native_lin_op) and y.default_backend.name == 'torch' and preconditioner and not all_available(y):
+    if not callable(native_lin_op) and is_sparse(native_lin_op) and y_tensor.default_backend.name == 'torch' and preconditioner and not all_available(y):
         warnings.warn(f"Preconditioners are not supported for sparse {method} in {y.default_backend} JIT mode. Disabling preconditioner. Use Jax or TensorFlow to enable preconditioners in JIT mode.", RuntimeWarning)
         preconditioner = None
     if not callable(native_lin_op) and is_sparse(native_lin_op) and not all_available(y) and not method.startswith('scipy-') and isinstance(preconditioner, IncompleteLU):