adding resnet18 support

[cl3m3nt]

Adding pre-trained Resnet18 model for transfer learning. Model has been trained using CIFAR100 coarse (20 super classes).

Changes are the following:

• adding keras_resnet18.py part (a full "manual implementation of resnet18 as it's not available from keras applications)
• updating keras.py part to load keras_resnet18 part & define Resnet18LinearKeras and Resnet18CategoricalKeras classes
• updating tflite.py part to optionally take into account inference on TPU Edge
• updating utils.py to take into account new model type (resnet18)
• updating train.py to take into account new model type (resnet18)

[DocGarbanzo]

Hi @cl3m3nt - do you still want to pursue this? We have a reset 18 in pytorch now, but there is no harm having this in tf, too?

[sctse999]

@cl3m3nt this looks great and I believe a lot of people would like to see this merged. Do you have time to work on this?

[tikurahul]

We already have Resnet18 with PyTorch now. FYI

[cl3m3nt]

Thanks @sctse999. Yes I could find some extra time to update it so that it meets @DocGarbanzo guidance. But I would also need to adapt taking into account last dev significant updates. I need to evaluate the effort before. Also I was considering making a PR to keras team so that Resnet18 would be available natively out from tf.keras.applications. Let me take some time to rethink about that.

[cl3m3nt]

We already have Resnet18 with PyTorch now. FYI

Thanks @tikurahul for the heads up. I'll figure out whether I can update anyway this PR so that Resnet18 is available from keras as well. Meanwhile, do you think that mobilenetv2 would make sense to be available ? Resnet18 got 7 Millions parameters vs Mobilenet which comes 2.2 Millions only. Might be interesting to have it as well to decrease latency when using Transfer Learning. What do you think ?

[sctse999]

Great I would love to see resnet18 or mobilenetv2 on keras as well.