fix(flux): warn on unsupported ip_adapter_masks

[akshan-main]

What does this PR do?

Addresses the flux review (#13584), Issue 1: FluxIPAdapterAttnProcessor accepts ip_adapter_masks but never applies them, so region-specific IP-Adapter conditioning silently has no effect (the repro in the issue prints 0.0 between masked and unmasked runs).

I looked at implementing the masks properly (the SD IPAdapterAttnProcessor2_0 pattern), but:

• no Flux pipeline plumbs ip_adapter_masks through, so there's no caller to exercise it, and
• the SD path applies masks spatially, while the Flux processor only sees packed-sequence latents and doesn't have the spatial shape needed to downsample/map a mask correctly.

So rather than ship a risky reimplementation against Flux's packed layout with no caller, this stops the silent no-op: passing ip_adapter_masks now emits a warning_once that it's unsupported and ignored. Full regional-mask support for Flux would be a follow-up (and needs spatial info plumbed in) — happy to take that on if you'd like it.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? (Part of [meta issue] Systematic model/pipeline review findings / tracking #13656, finding from flux model/pipeline review #13584) and discussed on slack with Sayak
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@hlky @sayakpaul @yiyixuxu

[github-actions]

Hi @akshan-main, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

[yiyixuxu]

I think we can just remove this