[chat] fix value computing bug

[zhang-yi-chi]

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🚨 Issue number

fixed #3622

📝 What does this PR do?

https://github.com/hpcaitech/ColossalAI/blob/b0ce5a10326912961f0bc07cbbd250bab7b9c399/applications/Chat/coati/models/base/critic.py#L45-L50

We should use the generation part of the sequenses to compute value. So the indexing should be [-num_actions:] instead of [:-num_actions] and using action_mask.

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