fix(model_manager): detect Flux 2 Klein LoRAs in Kohya format with transformer-only keys

[Pfannkuchensack]

Summary

LoRAs trained with musubi-tuner (and potentially other trainers) that only target transformer blocks (double_blocks/single_blocks) without embedding layers (txt_in/vector_in/context_embedder) were incorrectly classified as Flux 1.

The existing Kohya-format detection in _is_flux2_lora_state_dict and _get_flux2_lora_variant only checked for embedding layer keys (lora_unet_txt_in.*, lora_unet_vector_in.*, lora_unet_context_embedder.*). LoRAs that exclusively target transformer blocks (e.g. lora_unet_double_blocks_0_img_attn_proj.*) fell through all checks and defaulted to Flux 1.

This PR adds fallback detection for Kohya-format LoRAs using transformer block tensor dimensions:

• _is_flux2_lora_state_dict: Check img_attn_proj hidden_size (lora_down.weight shape[1]). If hidden_size ≠ 3072 → Flux 2. If hidden_size = 3072 (ambiguous between Flux 1 and Klein 4B), check MLP ratio via img_mlp_0 ffn_dim — Klein 4B uses mlp_ratio=6 (ffn_dim=18432) vs Flux 1 mlp_ratio=4 (ffn_dim=12288).
• _get_flux2_lora_variant: Same img_attn_proj hidden_size check to determine Klein 4B (3072) vs Klein 9B (4096).

Both functions also handle LoKR variants of these keys.

QA Instructions

Tested with simulated state dicts matching the tensor shapes from a real musubi-tuner Flux 2 Klein 9B LoRA:

State dict	_is_flux2_lora_state_dict	_get_flux2_lora_variant
Klein 9B (hidden_size=4096)	True	Klein9B
Klein 4B (hidden_size=3072, mlp_ratio=6)	True	Klein4B
Flux 1 (hidden_size=3072, mlp_ratio=4)	False	—

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
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• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)