[Docs] add note on caching in fast diffusion (#8675)
* add note on caching in fast diffusion
* formatting
* Update docs/source/en/tutorials/fast_diffusion.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>