Feat: fal flashpack support

[RakshithaKowlikar]

What does this PR do?

This PR adds optional Flashpack integration for significantly faster model weight loading.
Read this blog to understand what flashpack is.

• adds use_flashpack parameter to enable flashpack loading
• automatic graceful fallback to safetensors if flashpack is unavailable or incompatible
• supports variant parameter for specific weight files (e.g., model.fp16.flashpack)
• feature is optional via use_flashpack=True

Fixes # (12550)

Requires optional dependency: pip install flashpack

flashpack repository: https://github.com/fal-ai/flashpack

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.

[iwr-redmond]

Pinging to remove stale again.

See also #12999 (comment) from @DN6:

There's an existing PR (#12700) for FlashPack that is a bit further along and more closely matches our expectations for file loading.