multi_agent_type correct cost/usage calculations

[fatihozturkh2o]

• Fixes Fix cost for multi-agent #1838
• Now returns desired outputs like:

{'usage_including_cached_inference': {'total_cost': 0.040175,
  'gpt-4o-2024-05-13': {'cost': 0.040175,
   'prompt_tokens': 7021,
   'completion_tokens': 338,
   'total_tokens': 7359}},
 'usage_excluding_cached_inference': {'total_cost': 0.040175,
  'gpt-4o-2024-05-13': {'cost': 0.040175,
   'prompt_tokens': 7021,
   'completion_tokens': 338,
   'total_tokens': 7359}}}

Reference: https://github.com/microsoft/autogen/blob/63d3297ffc66120a16e60391af538354a11e5d35/autogen/agentchat/utils.py#L29

[pseudotensor]

thanks!