fix OpenAI Embedding length error (#1972)

### What problem does this PR solve?
 
#1958   fix OpenAI Embedding length error

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

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Co-authored-by: Zhedong Cen <cenzhedong2@126.com>

rag/llm/embedding_model.py

         self.model_name = model_name
 
     def encode(self, texts: list, batch_size=32):
-        texts = [truncate(t, 8196) for t in texts]
+        texts = [truncate(t, 8191) for t in texts]
         res = self.client.embeddings.create(input=texts,
                                             model=self.model_name)
         return np.array([d.embedding for d in res.data]
                         ), res.usage.total_tokens
 
     def encode_queries(self, text):
-        res = self.client.embeddings.create(input=[truncate(text, 8196)],
+        res = self.client.embeddings.create(input=[truncate(text, 8191)],
                                             model=self.model_name)
         return np.array(res.data[0].embedding), res.usage.total_tokens