Add dataset % sample num equally distribute

[Katehuuh]

What does this PR do?

Update #3829 to num_samples include % of dataset and equally distribute the selection.

Useful when the max number of num_samples is unknown.

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Edit1: untested yet Tested!

[Katehuuh]

Tested using pair QA dataset, 20epoch

sampleDiffAppleOrange.json:

[
    {
        "instruction": "Which fruit is preferred by Katehuuh?",
        "input": "",
        "output": "Katehuuh prefers apples."
    },
    {
        "instruction": "Can you tell me Katehuuh's favorite fruit?",
        "input": "",
        "output": "Orange is the favorite fruit of Katehuuh."
    },
...

dataset_info.json:

  "sampleDiffAppleOrange": {
    "file_name": "sampleDiffAppleOrange.json",
    "num_samples": "50%",
    "formatting": "alpaca"
  },

"num_samples": "50%" will skip one of the two sample so answer only apples.

[Katehuuh]

@hiyouga oi, mind checking PR? 😁