Update fid.py and eval.py

[Yunyung]

Hi 👋,

We think the num_workers parameter for DataLoader is important in some environments such as docker or limited RAM, otherwise causing failure. So we add num_workers when evaluating.

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Supplementary materials

For example:
In docker environment, the shared memory is limited causing specific error:

RuntimeError: DataLoader worker (pid xxxx) is killed by signal: Bus error. It is possible that dataloader's workers are out of shared memory. Please try to raise your shared memory limit.

References:
pytorch/pytorch#5040
ultralytics/yolov3#1151