Fix: adding import nececeary for using attention layers as bottlenecks

[rasmuspjohansson]

Adding the following import from fastai.layers import _get_norm in order for the following code to be able to run

def BatchNormZero(nf, ndim=2, **kwargs):\n", " "BatchNorm layer with nf features and ndim initialized depending on norm_type. Weights initialized to zero."\n", " return _get_norm('BatchNorm', nf, ndim, zero=True, **kwargs)\n",

After this fix it becomes possible to train a timm model based unet with attention or double_attention as bottleneck

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[muellerzr]

Thanks!