Adding test cases for the bugs found, local_rank None and output dir …

[albertodepaola]

What does this PR do?

Fixes 2 bugs introduced on #220:

1. None value was used in the json file name when the local_rank was not informed.
2. Directory structure was not created before saving the metrics json file.

Created new tests to reproduce these errors and fixed them

Testing

Running unit tests and running finetuning for an epoch:

1. Tests:
   

2. Finetuning command:
   python -m llama_recipes.finetuning --use_peft --peft_method lora --quantization --model_name ../llama/models_hf/7B/ --output_dir ../llama/models_ft/Llama-2-7b-peft-samsum-1 --batch_size_training 2 --gradient_accumulation_steps 2 --num_epochs 1 --save_model --save_metrics --dataset samsum_dataset

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?