fix: small bug fixes/improvements for pkg

[d-kleine]

Fixes for several issues in the package

• fixes #675; code for the encode function has been taken from ch05\07_gpt_to_llama\converting-llama2-to-llama3.ipynb (please double-check if everything is okay)
• fixes tqdm import
• adds weights_only=True to only load the weights with torch.load (set to False by default in torch versions < 2.6; avoiding mismatches and warnings)