PT: add uniform likelihood bucket batching

returnn/torch/data/pipeline.py

@@ -373,6 +373,33 @@ def __getitem__(self, index):
         raise Exception(f"{self.__class__.__name__}.__getitem__ is not supported")
 
 
+class UniformLikelihoodBucketOrderingIterDataPipe(BucketOrderingIterDataPipe):
+    def __iter__(self):
+        seq_lens = []
+        for batch in super().__iter__():
+            seq_lens.extend(int(d[self._length_key].shape[0]) for d in batch)
+            yield batch
+        seq_lens.sort()
+
+        new_max_seq_lens = [
+            max(1, v)
+            for v in [
+                *seq_lens[:: len(seq_lens) // len(self._max_seq_lens)][1:],
+                self._max_seq_lens[-1],  # keep old upper bound
+            ]
+        ]
+        old_batch_sizes = [seq_len * bsize for seq_len, bsize in zip(self._max_seq_lens, self._max_bucket_sizes)]
+        new_max_bucket_sizes = [
+            max(1, round(bsize / seq_len)) for seq_len, bsize in zip(new_max_seq_lens, old_batch_sizes)
+        ]
+
+        self._max_bucket_sizes = new_max_bucket_sizes
+        self._max_seq_lens = new_max_seq_lens
+
+        cfg_str = ", ".join(f"{limit}: {size}" for limit, size in zip(new_max_seq_lens, new_max_bucket_sizes))
+        print(f"optimized bucket batching configuration: {cfg_str}", file=log.v3)
+
+
 def get_batching_iterable_dataset_from_config(
     *, dataset: torch.utils.data.IterableDataset, config: Config, train: bool
 ) -> torch.utils.data.IterableDataset: