add EfficientNet for torchvision

[WZMIAOMIAO]

🚀 Feature

Add EfficientNet models into torchvision module.

Motivation

I very love this projects.
I noticed that in 2019 it was said that efficientnet models would be added. #980
But up to now, there is no efficientnet models.
I'm looking forward to adding more new models to torchvision.

Additional context

I refer to the official efficientnet model of tensorflow implemented with keras module link and the mobilenetV3 model in torchvision link. Then I built efficientnet models Test9_efficientNet and the conversion weight script trans_weights_to_pytorch.py.
If you think it's useful a little bit, I hope you can add efficientnet models to torchvision as soon as possible.

[pmeier]

Hey @WZMIAOMIAO. I've skimmed #980 and as far as I understood the problem is not the implementation, but rather the training. If you have more insights or ressources, I suggest you get in touch with the other participants in the discussion.

Anyway, since there already is an issue for this, lets keep the discussion there.