Add sequence2txt model.py (#1633)

### What problem does this PR solve?

#1514 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/llm/__init__.py

@@ -17,7 +17,7 @@ from .embedding_model import *
 from .chat_model import *
 from .cv_model import *
 from .rerank_model import *
-
+from .sequence2txt_model import *
 
 EmbeddingModel = {
     "Ollama": OllamaEmbed,
@@ -81,3 +81,12 @@ RerankModel = {
     "Youdao": YoudaoRerank,
     "Xinference": XInferenceRerank
 }
+
+
+Seq2txtModel = {
+    "OpenAI": GPTSeq2txt,
+    "Tongyi-Qianwen": QWenSeq2txt,
+    "Ollama": OllamaSeq2txt,
+    "Azure-OpenAI": AzureSeq2txt,
+    "Xinference": XinferenceSeq2txt
+}