[DRAFT] Handle None preds when no target channels for a stream

src/weathergen/utils/validation_io.py

@@ -60,6 +60,20 @@ def write_output(
             preds_s, targets_s, t_coords_s, t_times_s = [], [], [], []
             targets_lens[-1] += [[]]
 
+            # for the case where no target channels are defined
+            # for a stream, e.g., when only using it as input
+            # we produce None preds and we need to handle this case
+            if preds is None or targets is None:
+                num_channels = len(stream_info.get("val_target_channels", []))
+                ens_size = int(stream_info.get("pred_head", {}).get("ens_size", 1))
+
+                preds_all[-1] += [np.zeros((ens_size, 0, num_channels), dtype=np.float32)]
+                targets_all[-1] += [np.zeros((0, num_channels), dtype=np.float32)]
+                targets_coords_all[-1] += [np.zeros((0, 2), dtype=np.float32)]
+                targets_times_all[-1] += [np.array([], dtype="datetime64[ns]")]
+                targets_lens[-1][-1] = [0 for _ in range(batch_size)]
+                continue
+
             for i_batch, (pred, target) in enumerate(zip(preds, targets, strict=True)):
                 pred, target = pred.to(fp32), target.to(fp32)