model : support GLM 4.6 (make a few NextN/MTP tensors not required) (#16359)

model : support GLM 4.6 (make a few NextN/MTP tensors not required) (#16359)

* Make a few GLM tensors not required

layer.nextn.shared_head_head and layer.nextn.embed_tokens are both excluded from GLM 4.6 resulting in the model not loading after conversion/quantization, this marks those tensors as not required which makes it work

* Update llama-model.cpp

layer.nextn.shared_head_norm also not required in case of future models