Fix sampling method to handle all -inf scores in next_token_scores 
Author: Saibo-creator
Pull request: yes

This commit addresses a bug in the token generation process where next_token_scores are all -inf, leading to NaN probabilities after softmax. This typically happens in the constrained generation process where no tokens are allowed to be generated anymore. The fix involves adjusting probabilities in such scenarios to ensure pad_token_id has a probability of 1, thus enabling correct sampling and avoiding runtime errors. Also includes error handling for cases where pad_token_id is not defined but required.

# What does this PR do?

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Fixes #27676 #13707 #15169 


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Comments:
@Saibo-creator see related comment [here](https://github.com/huggingface/transformers/issues/27676#issuecomment-1831735403) :)

check PR #27797 which fixes the relevant issues