Fix loss mask when answer_only_loss=True
State: closed | Pull request: yes
Labels: name: Run CICD

# What does this PR do ?

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<!-- pylint-output-strict-mode-false -->

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

---

Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module nemo.collections.llm.gpt.data.packed_sequence
nemo/collections/llm/gpt/data/packed_sequence.py:94:0: C0301: Line too long (123/119) (line-too-long)
nemo/collections/llm/gpt/data/packed_sequence.py:89:0: C0115: Missing class docstring (missing-class-docstring)
************* Module nemo.utils.sequence_packing_utils
nemo/utils/sequence_packing_utils.py:53:0: C0301: Line too long (125/119) (line-too-long)
nemo/utils/sequence_packing_utils.py:112:0: C0301: Line too long (127/119) (line-too-long)
nemo/utils/sequence_packing_utils.py:121:0: C0301: Line too long (122/119) (line-too-long)
nemo/utils/sequence_packing_utils.py:122:0: C0301: Line too long (139/119) (line-too-long)
************* Module scripts.nlp_language_modeling.prepare_packed_ft_dataset
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:207:0: C0301: Line too long (157/119) (line-too-long)
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:170:0: C0115: Missing class docstring (missing-class-docstring)
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:176:4: C0116: Missing function or method docstring (missing-function-docstring)
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:189:0: C0116: Missing function or method docstring (missing-function-docstring)

-----------------------------------
Your code has been rated at 9.48/10
```

---

Thank you for improving NeMo's documentation!

[🤖]: Hi @ashors1 👋,

We wanted to let you know that a [CICD pipeline](https://github.com/NVIDIA/NeMo/actions/runs/12102356329) for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g