[Flux2] Fix LoRA loading for Flux2 Klein by adaptively enumerating transformer blocks (#13030)

* Resolve Flux2 Klein 4B/9B LoRA loading errors

* Apply style fixes

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Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

Author: 3 people

src/diffusers/loaders/lora_conversion_utils.py

@@ -2321,8 +2321,14 @@ def _convert_non_diffusers_flux2_lora_to_diffusers(state_dict):
     prefix = "diffusion_model."
     original_state_dict = {k[len(prefix) :]: v for k, v in state_dict.items()}
 
-    num_double_layers = 8
-    num_single_layers = 48
+    num_double_layers = 0
+    num_single_layers = 0
+    for key in original_state_dict.keys():
+        if key.startswith("single_blocks."):
+            num_single_layers = max(num_single_layers, int(key.split(".")[1]) + 1)
+        elif key.startswith("double_blocks."):
+            num_double_layers = max(num_double_layers, int(key.split(".")[1]) + 1)
+
     lora_keys = ("lora_A", "lora_B")
     attn_types = ("img_attn", "txt_attn")