[BUG] Fix build train valid test datasets (#8823)

Author: JunnYu

paddlenlp/data/causal_dataset.py

@@ -147,7 +147,9 @@ def build_train_valid_test_datasets(
  # Parse the values.
  output = get_datasets_weights_and_num_samples(data_prefix, train_val_test_num_samples)
  prefixes, weights, datasets_train_valid_test_num_samples = output
- train_num_samples, valid_num_samples, test_num_samples = map(sum, zip(*datasets_train_valid_test_num_samples))
+ # NOTE: megatron/gpt_dataset.py has been updated. When creating BlendableDataset, we will use the raw train_val_test_num_samples instead of the expanded ones.
+ # Please refer to https://github.com/NVIDIA/NeMo/blob/72f630d087d45655b1a069dc72debf01dfdbdb2d/nemo/collections/nlp/data/language_modeling/megatron/gpt_dataset.py#L74-L80 for more information
+ train_num_samples, valid_num_samples, test_num_samples = datasets_train_valid_test_num_samples
 
  # Build individual datasets.
  train_datasets = []