Keras team hasn't included resnet, resnet_v2 and resnext in the current module, they will be added from Keras 2.2.5, as mentioned here.
For a workaround, you can use keras_applications module directly to import all ResNet, ResNetV2 and ResNeXt models, as given below
from keras_applications.resnet import ResNet50
Or if you just want to use ResNet50
from keras.applications.resnet50 import ResNet50
Alternatively, you can always build from source as mentioned here.
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