remove extraneous default params for nims that expect conservative pa…

[leondz]

resolves #748

• add a "blocked params" value to OpenAICompatible so we can control verbosity of payloads
• strip out Nones from OpenAI API payloads
• force unset defaults for some params in nim

one question - why are these defaults defined twice?

    # template defaults optionally override when extending
    DEFAULT_PARAMS = Generator.DEFAULT_PARAMS | {
        "temperature": 0.7,
        "top_p": 1.0,
        "frequency_penalty": 0.0,
        "presence_penalty": 0.0,
        "stop": ["#", ";"],
        "blocked_params": set(),
    }

    temperature = 0.7
    top_p = 1.0
    frequency_penalty = 0.0
    presence_penalty = 0.0
    stop = ["#", ";"]
    blocked_params = set()

[leondz]

-- ideally would have liked to have done this via config, not sure it woulda been possible though