Potential incorrect reshaping in 2D positional embedding

### Describe the bug

Hi there,

I have concerns with this line of code (https://github.com/huggingface/diffusers/blob/main/src/diffusers/models/embeddings.py#L282).

Specifically, `grid_size` is the tuple consisting of the height `H` and width `W` of the image. `grid` computed in L280 should have the shape `2*H*W`, and L282 reshapes it into `2*1*W*H`. The dimensions `W*H` will be later flattened to match the dimensions of the latent.

However, if you continue to `PatchEmbed` (https://github.com/huggingface/diffusers/blob/main/src/diffusers/models/embeddings.py#L549), you will notice that the latent with shape `BCHW` is flattened into `B(H*W)C`, this flattening operation does not seem to match with `grid` in L282. I think this reordering will mess up the ordering of dimensions when being flattened in case `H` and `W` are not equal.

### Reproduction

This potential bug is conceptual and no need for reproduction.

### Logs

```shell

```

### System Info

Current `diffusers` implementation.

### Who can help?

@yiyixuxu @sayakpaul

Inizia in src/diffusers/models/embeddings.py, nella costruzione della griglia intorno alla riga 280 e nell’implementazione di PatchEmbed intorno alla riga 549. Confronta l’ordinamento della griglia posizionale con l’appiattimento da BCHW a B(H*W)C per altezza e larghezza diverse, quindi verifica se l’ordinamento corrisponde e aggiungi un test di regressione o una correzione se non corrisponde.

Scritto dal modello di indicizzazione a partire dal testo della issue.