Keep target_for_fewshot_sorting for another purpose

[sadra-barikbin]

• Removed task arg from FewShotSampler.init_fewshot_sampling_balanced() because it no longer needs it.
• Removed fewshot arg from task.doc_to_target() and doc.get_golds() as it's no longer used.
• Added get_target_for_fewshot_sorting() to Doc which gives target_for_fewshot_sorting if it's set otherwise gives the gold.
• Adopted some of the tasks in the code to the change to check its viability.

Closes #240

[NathanHB]

Hi ! Thanks for your PR, it appears the tests are not passing, I already took a look and it looks good but I will be reviewing more thourouly once the tests pass :)