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

Comienza en src/diffusers/models/embeddings.py, en la construcción de la cuadrícula alrededor de la línea 280 y en la implementación de PatchEmbed alrededor de la línea 549. Compara el orden de la cuadrícula posicional con el aplanado de BCHW a B(H*W)C para alturas y anchuras desiguales; después, confirma si el orden coincide y añade una prueba de regresión o una corrección si no es así.

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