[Fix: pixart-alpha] random 512px resolution bug (#5842)
* [Fix: pixart-alpha]
add ASPECT_RATIO_512_BIN in use_resolution_binning for random 512px image generation.
* add slow test file for 512px generation without resolution binning
* fix: slow tests for resolution binning.
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Co-authored-by: jschen <chenjunsong4@h-partners.com>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
