MultiProcDataset, random_seed_offset from parent

The default random seed offset logic
might not work properly in the subproc,
e.g. accessing the distributed environ.

returnn/datasets/multi_proc.py

@@ -48,6 +48,8 @@ def __init__(
         for k, v in kwargs.items():
             if k not in dataset:
                 dataset[k] = v
+        if "random_seed_offset" not in dataset:
+            dataset["random_seed_offset"] = self.random_seed_offset
         self.dataset = dataset
         self.num_workers = num_workers
         self.buffer_size = buffer_size