Hm, not sure why the loop would be a problem. The heaving lifting is still done under the hood by PyTorch. I would also assume that when you give a sequence the nn.LSTM that internally it also just loops over each item.
Maybe another alternative – again, I haven’t tried it:

Treat a sequence of images as a batch of images and push it through the CNN
Treat the batch of encoded images again as a sequence and push it in one go through the LSTM (no loop required)

Not idea how to extend it to support batches of images sequences, or if it possible at all.