Enhance input validation in sampling script

[mandlinsarah]

This update introduces input validation for the command-line arguments to ensure that all necessary files are provided and accessible, preventing runtime errors and enhancing user experience.

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[Gopi-Uppari]

Hi @mandlinsarah,

Looks like this change encounters the error: 'ValueError: Checkpoint file not found: /content/gemma/2b/' , if i tried to provide the correct model checkpoint and tokenizer paths. Could you please review your modified code again for all cases.
Please refer to this gist file.

Thank you.