CombinedDataset: allow sub-dataset estimated_num_seqs not set

Author: patrick-wilken

returnn/datasets/meta.py

@@ -863,8 +863,9 @@ def __init__(self,
     # This will only initialize datasets needed for features occurring in data_map
     self.datasets = {key: init_dataset(datasets[key]) for key in self.dataset_keys}
 
-    self._estimated_num_seqs = sum([self.datasets[k].estimated_num_seqs for k in sorted(self.datasets.keys())])
     self.estimated_num_seq_per_subset = [self.datasets[k].estimated_num_seqs for k in sorted(self.datasets.keys())]
+    if all(num_seq is not None for num_seq in self.estimated_num_seq_per_subset):
+        self._estimated_num_seqs = sum(self.estimated_num_seq_per_subset)
 
     if data_dims:
       data_dims = convert_data_dims(data_dims)