Add litellm Demo - call cohere models using chatGPT I/O format

[ishaan-jaff]

I'm the maintainer of litellm https://github.com/BerriAI/litellm - a simple & light package to call Cohere, Anthropic, Azure, OpenAI LLM Models using chatGPT input/output format

This PR adds an example on how to use it

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

from litellm import completion

## 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)

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

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

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[ishaan-jaff]

@mrmer1 @co-sylvie can you please take a look at this PR. It simplifies making cohere api calls 😊

[ishaan-jaff]

Hi @mrmer1 @jalammar any update on getting this reviewed ?