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
 }
 ]
}