Implement `logprobs`

[abetlen]

logprobs return format should match OpenAI API. Currently calling a Llama instance with logprobs enabled just returns a list of floats.

Example of the correct format:

"logprobs": {
    "text_offset": [
        11,
        12,
        13,
        14,
        15,
        17,
        18,
        20,
        21,
        23,
        24,
        26,
        27,
        29,
        30,
        32
    ],
    "token_logprobs": [
        -0.028534053,
        -0.0013638621,
        -0.0001191709,
        -0.037809037,
        -0.008346983,
        -1.3900239e-05,
        -6.0395385e-05,
        -2.462996e-05,
        -5.4432137e-05,
        -4.3108244e-05,
        -6.0395385e-05,
        -4.382537e-05,
        -4.489638e-05,
        -4.751897e-05,
        -0.00017937786,
        -7.314978e-05
    ],
    "tokens": [
        "\n",
        "\n",
        "1",
        ",",
        " 2",
        ",",
        " 3",
        ",",
        " 4",
        ",",
        " 5",
        ",",
        " 6",
        ",",
        " 7",
        ","
    ],
    "top_logprobs": [
        {
            "\n": -0.028534053,
            "\n\n": -5.3414392,
            " (": -6.8118296,
            " in": -4.9322805,
            ":": -5.6061873
        },
        {
            "\n": -0.0013638621,
            " \u00a7\u00a7": -8.594428,
            "//": -9.296644,
            "1": -9.727121,
            "Count": -9.291412
        },
        {
            " 1": -10.996209,
            "\"": -12.673454,
            "#": -12.253096,
            "1": -0.0001191709,
            "One": -9.39247
        },
        {
            " -": -6.4947214,
            " 2": -7.7675867,
            ")": -8.327954,
            ",": -0.037809037,
            ".": -3.3655276
        },
        {
            "\n": -14.826643,
            " ": -10.675518,
            " 2": -0.008346983,
            " two": -16.126537,
            "2": -4.792885
        },
        {
            " ,": -11.469002,
            " 3": -12.7872095,
            ",": -1.3900239e-05,
            ".": -14.724538,
            "<|endoftext|>": -15.308233
        },
        {
            " ": -12.118958,
            " 3": -6.0395385e-05,
            " three": -17.906118,
            "3": -9.814757,
            "<|endoftext|>": -15.049129
        },
        {
            " ,": -10.729593,
            " 4": -14.016008,
            ",": -2.462996e-05,
            ".": -14.297305,
            "<|endoftext|>": -13.67176
        },
        {
            " ": -11.351273,
            " 4": -5.4432137e-05,
            "4": -10.086686,
            "<|endoftext|>": -13.919009,
            "\u00a0": -16.80569
        },
        {
            " ,": -10.206355,
            " 5": -12.87644,
            ",": -4.3108244e-05,
            ".": -13.588498,
            "<|endoftext|>": -13.03574
        },
        {
            " ": -11.478045,
            " 5": -6.0395385e-05,
            "5": -9.931537,
            "<|endoftext|>": -13.568035,
            "\u00a0": -16.266188
        },
        {
            " ,": -10.160495,
            " 6": -12.964705,
            ",": -4.382537e-05,
            ".": -14.101328,
            "<|endoftext|>": -13.08568
        },
        {
            " ": -11.344849,
            " 6": -4.489638e-05,
            "6": -10.329956,
            "<|endoftext|>": -14.879237,
            "\u00a0": -16.98358
        },
        {
            " ,": -10.096309,
            " 7": -12.389179,
            ",": -4.751897e-05,
            ".": -13.817777,
            "<|endoftext|>": -13.860558
        },
        {
            " ": -11.630913,
            " 7": -0.00017937786,
            " seven": -16.613815,
            "7": -8.680304,
            "<|endoftext|>": -14.859097
        },
        {
            " ,": -9.754253,
            " 8": -11.516983,
            ",": -7.314978e-05,
            ".": -13.250221,
            "<|endoftext|>": -12.703088
        }
    ]
}