Add bounds check for AuraFlow positional embedding indices

[Mr-Neutr0n]

Summary

• Add a bounds check in AuraFlowPatchEmbed.pe_selection_index_based_on_dim() to prevent out-of-bounds positional embedding indices
• When the input latent dimensions exceed the PE grid size, the centered-crop index calculation produces negative or out-of-range indices, causing a fatal CUDA assertion error (ind >= 0 && ind < ind_dim_size) that destroys the CUDA context for the entire process
• Raise a clear ValueError with guidance on how to fix the issue, instead of silently producing invalid indices

Fixes #12656

Test plan

• Verify that inputs within the PE grid size work as before
• Verify that inputs exceeding the PE grid raise a clear ValueError

[Mr-Neutr0n]

Friendly bump! Let me know if there's anything I should update or improve to help move this forward.