Add Moving MNIST dataset

[tsugumi-sys]

🚀 The feature

I would like to add MovingMNIST dataset.

Motivation, pitch

A lot of people uses Moving MNIST dataset for evaluating their proposed models according to PaperWithCode.

Alternatives

No response

Additional context

I've implemented Movning MNIST dataset in my personal project. So I would like to make a PR based on this.

I checked #3562 and #2690.

@pmeier @yassineAlouini
From this comment in #2690, I might should add to torchvision/prototype/datasets/_builtin. So where should I add Moving MNIST dataset ?

cc @pmeier

[pmeier]

Hey @tsugumi-sys, sorry for the late reply. Yes, we would accept a PR adding the dataset! Fortunately for this, the prototype datasets are currently on hold and so you can add it to torchvision.datasets directly. Meaning, we can probably re-use most of what you already implemented.

I've skimmed your linked implementation and it seems this is going into the right direction. In #2690 (review) I had quite a few concerns regarding the first draft of the other PR. Please make sure to address them on yours.