[NeMo-UX] Match nemo 1's default behavior for drop_last and pad_samples_to_global_batch_size
This PR updates the default behavior of NeMo-UX's dataloader to match NeMo 1's. See https://github.com/NVIDIA/NeMo/blob/d4d03857f432a9248e6d58ecea12b17870e112eb/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py#L1552-L1579.

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