Fix progress bar in Stable Diffusion pipeline

[neverix]

Describe the bug

Generating images with StableDiffusionPipeline does not display the total number of iterations because of tqdm and enumerate being swapped in the code.

This is a well-known issue that has at least two PRs proposed to fix it (#236, #242).

Reproduction

See Stable Diffusion colab notebook (link).

Logs

No response

System Info

`main` branch, Python 3.7