Adding documentation for packed dataset preparation with context para…
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Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module prepare_packed_ft_dataset
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:221:0: C0301: Line too long (157/119) (line-too-long)
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:184:0: C0115: Missing class docstring (missing-class-docstring)
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:190:4: C0116: Missing function or method docstring (missing-function-docstring)
scripts/nlp_language_modeling/prepare_packed_ft_dataset.py:203:0: C0116: Missing function or method docstring (missing-function-docstring)

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Your code has been rated at 9.44/10
```

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Thank you for improving NeMo's documentation!
# What does this PR do ?

This PR adds some documentation in the code for packed dataset preparation with context parallel.
  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [x] Documentation


