Turn duplicate code sections in training/testing loops into callable functions in utls

[mjsonofharry]

There's loads of identical code that could be extracted.

Write tests BEFORE extracting into functions.

[mjsonofharry]

The duplicate code is for getting clips from a single video. e.g.

            for j in range(0, batch_x.shape[0], CLIP_LENGTH):
                x_clip = batch_x[j:j+CLIP_LENGTH]
                y_clip = batch_y[j:j+CLIP_LENGTH]

[mjsonofharry]

If padding clips is implemented then it should automatically check if the clip length is too short and then yield a padded clip.