[model] Add support for Llama2, Palm, Cohere, Replicate Models - using litellm

[ishaan-jaff]

This PR adds support for models from all the above mentioned providers using https://github.com/BerriAI/litellm/

Here's a sample of how it's used:

from litellm import completion, acompletion

## set ENV variables
# ENV variables can be set in .env file, too. Example in .env.example
os.environ["OPENAI_API_KEY"] = "openai key"
os.environ["COHERE_API_KEY"] = "cohere key"

messages = [{ "content": "Hello, how are you?","role": "user"}]

# openai call
response = completion(model="gpt-3.5-turbo", messages=messages)

# llama2 call
model_name = "replicate/llama-2-70b-chat:2c1608e18606fad2812020dc541930f2d0495ce32eee50074220b87300bc16e1"
response = completion(model_name, messages)

# cohere call
response = completion("command-nightly", messages)

# anthropic call
response = completion(model="claude-instant-1", messages=messages)

When you open a pull-request, please be sure to include the following

• A descriptive title: [xxx] XXXX
• A detailed description
• Assign an issue type tag (label):
  • dataset (mimic-it download, usage, etc.),
  • demo (online demo),
  • doc (readme, wiki, paper, video, etc.),
  • evaluation (evaluation result, performance of Otter, etc.),
  • model (model configuration, components, etc.),
  • train (training configuration, process, code, etc.)

Thank you for your contributions!

[ishaan-jaff]

@ZhangYuanhan-AI @Luodian can I get a review on this PR 😊 ? Happy to add more tests/docs if needed

[Luodian]

Thanks for this PR! I will review it later and then consider merging it~

[Luodian]

@ishaan-jaff Thanks! The litellm looks soo cool. I am merging it now.