Add more CNN networks for encoding generation, allow custom model addition

✨ New features and improvements

• Add more CNN networks apart from the default MobileNetv3- the new networks added are: EfficientNet and ViT #190
• Allow user to define and plugin their own custom CNN models for encoding generation - an example of how to use this can be seen here: imagededup/examples/use_custom_model.ipynb #190
• Make encoding generation quicker and memory-efficient by removal of torch grad calculation #192

🔴 Bug fixes

• Update CNN link in readme to point to MobilenetV3 paper.
• Remove 'pretrained' parameter of mobilenetv3 to get rid of deprecation warning and use 'weights' parameter instead.
• Some typos.