dpo pad fix if none

Signed-off-by: Terry Kong <terryk@nvidia.com>

nemo_aligner/algorithms/dpo.py

@@ -53,7 +53,8 @@ def dpo_custom_collate(
     This collate happens outside of the torch data loader and is not compatible with the multiprocessing
     logic due to requiring communication collectives.
     """
-    assert pad_length_to_multiple_of >= 0, f"{pad_length_to_multiple_of=} must be >=0"
+    if pad_length_to_multiple_of is not None and pad_length_to_multiple_of < 0:
+        raise ValueError(f"{pad_length_to_multiple_of=} must be >= 0")
     chosen_tokens = [item["chosen"] for item in batch]
     rejected_tokens = [item["rejected"] for item in batch]
     chosen_lengths = torch.LongTensor([item["chosen_length"] for item in batch])