To use the model for inference in fp16 you should call model.half() after loading it.
Note that calling half puts all models weights in fp16, but in mixed precision training some parts are still kept in fp32 for stability (like softmax layers), so it might be a better idea to use amp in 01 opt mode instead of calling half.
I’m not sure if torch.cuda.amp.autocast()  helps in inference, better to ask this on pytorch forum