Add Support for Multi-GPU Training with PyTorch Lightning

[OrjwanZaafarani]

This pull request introduces support for multi-GPU training in the Sentence Transformers library using PyTorch Lightning.

The following changes have been made:

• Updated README.md to include instructions on how to perform multi-GPU training.
• Added a new module, SentenceTransformerMultiGPU.py, to enable multi-GPU training.
• Included PyTorch Lightning in the requirements.txt file to ensure compatibility.

This enhancement will allow users to leverage the power of multiple GPUs for faster and more efficient training with Sentence Transformers.

Closes the following issues:

• Multi-GPU training #311
• Multi-GPU training #556
• How to use multi GPU in training #59
• Multi-gpu fine-tunning of sbert with roberta-base #1898
• SentenceBert Fit-Method: Capture loss, Gradient Accumulation and Multi-GPU #1340
• MultiGPU training not avalilable on Sentence Transformer #1429