Update README with pre-requisites for Llama models

[Adibakhan14]

This Pull Request adds a new Pre-requisites section to the README file to help users set up the environment effectively before using the Llama models. The section includes:

Python version requirements to avoid compatibility issues.
PyTorch and CUDA requirements for model inference and fine-tuning.
GPU memory recommendations based on the model size (7B, 13B, 70B).
Mention of required tools (wget and md5sum) for downloading model weights.

Reason for Change:
The addition of the Pre-requisites section ensures that users have all necessary information upfront, reducing potential setup errors and providing clarity on hardware and software dependencies.

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