Issue with github api rate limiting

[iceisfun]

It seems that utils/google_utils.py has an issue in attempt_download after updating to the latest v4 stuff.

os.path.isfile is checked after calls to github, this ends up causing exceptions when a cluster of machines start making this api call and end up with rate limiting exceptions.

this function probably just needs to check if weights exists before making the api call.

# Don't download if we already have this file
if os.path.isfile(weights):
    return

before

response = requests.get('https://api.github.com/repos/ultralytics/yolov5/releases/latest').json()  # github api

Keep up the good work on yolov5

[github-actions]

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[glenn-jocher]

@iceisfun good observation. I'd noticed intermitent CI failures since this the v4.0 release which includes this update, this is likely the reason. There is a .exists() check in the function, I suppose we simply need to move it to before the API call. I'll look into this.