I believe that it is very necessary to run prediction on multiple inputs rather than just one. I’m facing the same problem (on computer vision) and found several similar topics.
https://forums.fast.ai/t/inference-on-test-images/29954
https://forums.fast.ai/t/is-there-a-way-to-get-predictions-against-future-test-datasets-not-available-during-training/30340
https://forums.fast.ai/t/how-to-add-a-test-set-to-an-existing-databunch/30410
Hope we will get a straight forward API for that rather than workaround for fastai 