Fix hf_transformers backend

[levilovearch]

The conversational pipeline has been removed since transformers 4.42 [1], and we need to use the text-generation pipeline instead [2]. I copied some code from backends/openai.py as the new pipeline is similar to Openai API in organising previous messages. The modified code has passed all related tests.
In addition, maybe it's better to change the class name TransformersConversational to something else. Please give me some suggestions, thanks.

Levi

Reference:

1. Remove ConversationalPipeline and Conversation object huggingface/transformers#31165
2. https://huggingface.co/docs/transformers/main/en/conversations

[LukeLIN-web]

Thank you for your effort! It is really helpful for me.