Fixed bug in argument parsing

[andrearosasco]

Without the parser argument added in this pull request--distributed can't be passed to the scripts.
This causes commands such as

python src/eval_retrieval.py \
    --openai-pretrained \
    --resume /path/to/checkpoints \
    --eval-mode $data_name \ ## replace with coco, imgnet, or cirr
    --gpu $gpu_id
    --model ViT-L/14

to fail when this function is called.

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