Use llama2-instruct style prompting

this also works well with mistral-7b-instruct

See https://github.com/facebookresearch/llama/blob/v2/llama/generation.py

src/chap/backends/llama_cpp.py

@@ -17,6 +17,11 @@ class Parameters:
  url: str = "http://localhost:8080/completion"
  """The URL of a llama.cpp server's completion endpoint."""
 
+ start_prompt: str = """<s>[INST] <<SYS>>\n"""
+ after_system: str = "\n<</SYS>>\n\n"
+ after_user: str = """ [/INST] """
+ after_assistant: str = """ </s><s>[INST] """
+
  def __init__(self):
  self.parameters = self.Parameters()
 
@@ -26,19 +31,19 @@ def __init__(self):
 
  def make_full_query(self, messages, max_query_size):
  del messages[1:-max_query_size]
- rows = []
+ result = [self.parameters.start_prompt]
  for m in messages:
  content = (m.content or "").strip()
  if not content:
  continue
+ result.append(content)
  if m.role == "system":
- rows.append(f"ASSISTANT'S RULE: {content}\n")
+ result.append(self.parameters.after_system)
  elif m.role == "assistant":
- rows.append(f"ASSISTANT: {content}\n")
+ result.append(self.parameters.after_assistant)
  elif m.role == "user":
- rows.append(f"USER: {content}")
- rows.append("ASSISTANT: ")
- full_query = ("\n".join(rows)).rstrip()
+ result.append(self.parameters.after_user)
+ full_query = "".join(result)
  return full_query
 
  async def aask(