Download model if not exists (New Feature)

[mfatiho]

If the vgg model is not in the chekcpoints directory, it is downloaded. To do this, the addresses in the model_url variable in vgg were used.

[justinkay]

Thanks for submitting these. Saved me some time getting started.

[mfatiho]

Thanks for submitting these. Saved me some time getting started.

You're welcome. I hope the repo manager will approve the pull requests as soon as possible so that it will be more useful.

[raffelbroox]

Thank you, it's really help me