Add basic arithmetic task

[yulinggu-cs]

Added basic_arithmetic as downstream evaluation task, example instance:

 {"id": "q85_1d1d_max1d_plus", "question": "Calculate 2 + 5 =",
    "choices": {"text": ["8", "7", "6", "17"], "label": ["A", "B", "C", "D"]},
    "answerKey": "B", "type_tag": "easy"}

To include in a training config, use:

  - label: basic_arithmetic
    type: downstream