If you want to activate FP16 with fastai, you add the command to_fp16() when you create your learner, not when you run it.
As in learn = create_cnn(data, models.resnet34, metrics=error_rate).to_fp16()
edit: this was done on a 1080Ti, prior to receiving my RTX 2070. It doesn’t work for the 2070 as it crashes my kernel.