graph: guard iswa kq_mask on its own buffer (ggml-org#24294)

A SWA-only draft head (e.g. StepFun MTP) leaves the base sub-cache
empty, so its kq_mask buffer stays null and asserts at load. Guard
each mask on its own buffer in set_input and can_reuse, base and swa.

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

Author: ServeurpersoCom

src/llama-graph.cpp

@@ -567,15 +567,20 @@ void llm_graph_input_attn_kv_iswa::set_input(const llama_ubatch * ubatch) {
         mctx->get_base()->set_input_v_idxs(self_v_idxs, ubatch);
     }
 
-    mctx->get_base()->set_input_kq_mask(self_kq_mask, ubatch, cparams.causal_attn);
+    // the kq mask guards on its own buffer: shared cells leave idxs unbacked while the mask stays live
+    if (self_kq_mask && self_kq_mask->buffer) {
+        mctx->get_base()->set_input_kq_mask(self_kq_mask, ubatch, cparams.causal_attn);
+    }
 
     // swa tensors may not be allocated if there are no SWA attention layers
     if (self_k_idxs_swa && self_k_idxs_swa->buffer) {
         mctx->get_swa()->set_input_k_idxs(self_k_idxs_swa, ubatch);
         mctx->get_swa()->set_input_v_idxs(self_v_idxs_swa, ubatch);
     }
 
-    mctx->get_swa()->set_input_kq_mask(self_kq_mask_swa, ubatch, cparams.causal_attn);
+    if (self_kq_mask_swa && self_kq_mask_swa->buffer) {
+        mctx->get_swa()->set_input_kq_mask(self_kq_mask_swa, ubatch, cparams.causal_attn);
+    }
 
     if (self_k_rot) {
         mctx->get_base()->set_input_k_rot(self_k_rot);
@@ -607,15 +612,19 @@ bool llm_graph_input_attn_kv_iswa::can_reuse(const llm_graph_params & params) {
       //res &= self_v_idxs->ne[0] == params.ubatch.n_tokens; // TODO: need to move this to the unified cache and check there
     }
 
-    res &= can_reuse_kq_mask(self_kq_mask, mctx->get_base(), params.ubatch, params.cparams);
+    if (self_kq_mask && self_kq_mask->buffer) {
+        res &= can_reuse_kq_mask(self_kq_mask, mctx->get_base(), params.ubatch, params.cparams);
+    }
 
     // swa tensors may not be allocated if there are no SWA attention layers
     if (self_k_idxs_swa && self_k_idxs_swa->buffer) {
         res &= self_k_idxs_swa->ne[0] == params.ubatch.n_tokens;
       //res &= self_v_idxs_swa->ne[0] == params.ubatch.n_tokens; // TODO: need to move this to the unified cache and check there
     }
 
-    res &= can_reuse_kq_mask(self_kq_mask_swa, mctx->get_swa(), params.ubatch, params.cparams);
+    if (self_kq_mask_swa && self_kq_mask_swa->buffer) {
+        res &= can_reuse_kq_mask(self_kq_mask_swa, mctx->get_swa(), params.ubatch, params.cparams);
+    }
 
     return res;
 }