Update together model prompt and stop sequence

[insop]

Update together model prompt and stop sequence

- update prompt based on the model name
- update stop sequence

TEST:

• the following models are tested using intro.ipynb
• completion model

model="mistralai/Mistral-7B-v0.1"
turbo = dspy.Together(model=model)

• instruction mode

model="mistralai/Mistral-7B-Instruct-v0.2"
turbo = dspy.Together(model=model)