Update finetuning_llama2_7b_ptl.rst

[bevhanno]

Update readme in fine-tuning llama2 tutorial for neuronx-distributed

Description:

The current tutorial suggest to ssh into a compute instance to run the checkpoint conversion. This requires to start a compute instance manually. The suggested edit uses an sbatch job that executes the conversion command on a node that will be started automatically for that purpose.

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