Fix and restrict NVFP4 edge-cases in llama-graph (#24331)

• Move post-GEMM MUL required for dequant b4 lora and bias add

see https://github.com/ggml-org/llama.cpp/pull/23484 :

1. For lora, I would presume we want fully dequantized values before doing the residuals, but this depends on how the LORAs were generated. Literature tells me LORA happens post-mul but pre-bias add https://github.com/ggml-org/llama.cpp/pull/8332
2. For ModelOPT, bias-add should happen on fully-dequantized values

• Restrict build_ffn for NVFP4 to supported combinations