Packed Sequence
jenkins jenkins
# What does this PR do ?

Support training with packed sequences for SFT and PEFT.
In this scenario, sequences in a batch are "concatenated" in the sequence length dimension, instead of "stacked" in a batch dimension. 
This is shown to significantly improve the training throughput.
Packed sequences will need to be prepared in a particular way (script in #8682 )

**Collection**: NLP

# Changelog 
- new GPTSFTPackedDataset class to handle the new dataset format
- pass `cu_seqlens` and `qkv_format` to the forward() function of mcore model

# Usage
Note: only train_ds is packed. Validation and test datasets are unchanged.
```python
python /opt/NeMo/examples/nlp/language_modeling/tuning/megatron_gpt_peft_tuning.py \
...
+model.data.train_ds.packed_sequence=True
```

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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
