fix: avoid crash on sdxl loras

[wbruna]

Some SDXL LoRAs (eg. PCM) can exceed 12k nodes.

Also tested with DMD2 4 step:

stable-diffusion.cpp/ggml/src/ggml.c:5764: GGML_ASSERT(cgraph->n_nodes < cgraph->size) failed
(...)
#12 0x000055769307cfa3 in ggml_build_forward_expand ()
#13 0x0000557692f0ff56 in LoraModel::build_lora_graph(std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ggml_tensor*, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ggml_tensor*> > >, SDVersion) ()

Before the assertion, the LoRA graph reaches 12608 nodes.

[LostRuins]

Related: 1be2491#r155465406

[DelusionalLogic]

I ran into the problem too, and increasing the node limit didn't seem to have any ill effects (other than the increased memory usage i guess). I can't help but wonder if there's not some way to calculate the required buffer size based on the input instead of leaving it as a static define.