Update examples to give clear error messages

[aakashapoorv]

This PR updates the example scripts to confirms that the max_seq_len is within an acceptable range (1 <= max_seq_len <= 8192), and the given ckpt_dir (checkpoint directory), tokenizer_path (tokenizer file paths) exist and gives clear error messages.

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[subramen]

Thanks @aakashapoorv I think it might be better to add the asserts in the build function instead of in the example scripts. https://github.com/meta-llama/llama3/blob/cc44ca2e1c269f0e56e6926d7f4837c983c060dc/llama/generation.py#L37

[aakashapoorv]

@subramen Thanks for the suggestion! Thats better, I've moved the assertions into the build function of the Llama class 😊