Fix missing import in BLIP captioning

[paboum]

Resolving #1820

[oumad]

I was about to fork the project to do this, thank you,
I also checked in sd-scripts they don't have this issue. It's only recently in this repo, after the merge, the url check was added in finetune/make_captions.py

        if not is_url(args.caption_weights) and not os.path.isfile(args.caption_weights):
            args.caption_weights = os.path.join("..", args.caption_weights)

but not in the imports.

from blip.blip import blip_decoder

[paboum]

Seems to have been properly merged in d653e59 but later lost.
Perhaps @CjangCjengh could see why.

[bmaltais]

Oh... somehow it was removed from my side... OK, I will merge this in the dev branch. checkout that branch to get the updated code.