follow-up: move Idefics2 to a shared location to eliminate unexpected dependency.

[lifuhuang]

Motivation

Quick follow up from #6494

Idefics2 is a vision model produced by HuggingFace, which is shared by both MiniCPMV, Phi4MM, and many other VLMs.
Currently Phi4MM is taking an incorrect dependency on MiniCPMV module, only because Idefics2 was placed there.

Modifications

• Extract Idefics from MiniCPMV into a separate module to remove.
• Add file headers to phi4mm following SGL convention.

Checklist

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