[NPUW][OpenVINO 2025.4] Hot fix for Speculative Decode (#33062)

### Details:
 - *Sibling of #33061
- *Fixes speculative decode acceptance rate by allowing trim of draft
model*

### Tickets:
 - *EISW-193919*

Author: AsyaPronina

src/plugins/intel_npu/src/plugin/npuw/llm_infer_request.cpp

@@ -523,8 +523,8 @@ void ov::npuw::LLMInferRequest::update_kvcache_for(
 
 void ov::npuw::LLMInferRequest::trim_kvcache_for_speculative_decoding(ov::SoPtr<ov::ITensor> position_ids) {
     auto& kvcache_desc = m_npuw_llm_compiled_model->m_kvcache_desc;
-    // FIXME: It can not work with OmniThinker for now.
-    OPENVINO_ASSERT((position_ids->get_shape().size() >= 2) && (position_ids->get_shape().back() >= 1));
+    // FIXME: It won't work with Qwen2.5-VL/Omni for now.
+    OPENVINO_ASSERT((position_ids->get_shape().size() == 2) && (position_ids->get_shape().back() >= 1));
     auto position_id = position_ids->data<int64_t>()[0];
     auto dirty_num = kvcache_desc.num_stored_tokens - static_cast<uint32_t>(position_id);
     if (dirty_num > 0) {
@@ -926,15 +926,16 @@ void ov::npuw::LLMInferRequest::infer() {
     //    number of logits.
     // The outcome of two items is that prefill and generate stages
     //    can be safely differentiated by start position id for
-    //    both main and draft models.
+    //    both main and draft models for most of LLMs.
     if (input_ids->get_shape()[layer_ids::INPUT_IDS_SEQ_LEN_DIM] > 1 &&
         position_ids->data<int64_t>()[0] == m_first_position_id) {
         infer_prefill(input_ids, attention_mask, position_ids, token_type_ids);
     } else {
-        auto& kvcache_desc = m_npuw_llm_compiled_model->m_kvcache_desc;
-        // Need to reconsider the solution. Some model like Qwen2.5VL, doesn't use speculative decoding,
-        // but it may have repeated position ids, then it will trigger kvcache trim and cause AC issue.
-        if (kvcache_desc.max_generation_token_len > 1) {
+        // FIXME: Need to make the solution smarter.
+        // Qwen2.5VL uses 3D position_ids but current `trim_kvcache_for_speculative_decoding`
+        // doesn't take this into account and causes accuracy issues.
+        // Speculative Decode isn't supposed to work with such position_ids currently.
+        if (position_ids->get_shape().size() < 3) {
             trim_kvcache_for_speculative_decoding(position_ids);
         }
         infer_generate(input_ids, attention_mask, position_ids, token_type_ids);