Akoumparouli/low mem mixtral ckpt converter
jenkins jenkins
# What does this PR do ?
This add the option --low-ram to enable checkpoint conversion in low-ram computers for large checkpoints (e.g. 250GB).

Memory reduction is achieved by not having the full converted checkpoint and the huggingface checkpoint on memory at the same time. Instead, it saves the converted checkpoint layer by layer, remove the HF checkpoint from memory and finally restores the converted layers and saves to .nemo format.

**Collection**: NLP

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage
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```python
# Add a code snippet demonstrating how to use this 
```

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- [ ] New Feature
- [ ] Bugfix
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# Additional Information
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