mtp: support for gemma-4 E2B and E4B assistants (#24282)

* models: update converter to support smaller assistants

* models: add masked_embd tensors to gemma4-assist arch

* gemma-4: remove temp debug for conversion

* gemma-4-mtp: filter out masked_embedding tensors during conversion

src/llama-arch.h

@@ -566,8 +566,11 @@ enum llm_tensor {
     LLM_TENSOR_NEXTN_HNORM,
     LLM_TENSOR_NEXTN_SHARED_HEAD_HEAD,
     LLM_TENSOR_NEXTN_SHARED_HEAD_NORM,
+    LLM_TENSOR_MASKED_EMBD_CENTROIDS,
+    LLM_TENSOR_MASKED_EMBD_ORDERING,
 };
 
+
 enum llm_tensor_layer {
     LLM_TENSOR_LAYER_INPUT,
     LLM_TENSOR_LAYER_REPEATING,