Add EfficientNet model for PyTorch
# What does this PR do?
This PR adds the EfficientNet model family to HuggingFace transformers proposed in #15759 (PyTorch only for this PR).
The implementation is based on that of Ross Wightman's pytorch-image-models [implementation](https://github.com/rwightman/pytorch-image-models/blob/master/timm/models/efficientnet.py).

## Who can review?
@NielsRogge 
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Working on this.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.