[NPUW] Fix LLMInferRequest to work with the input prompt of len 1 (#32267)

### Details:
 - *Fix LLM inference on NPU for input prompt of length 1*
 

### Tickets:
 - *N/A*

Author: AsyaPronina

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

@@ -629,13 +629,16 @@ void slice_out_embeds(std::shared_ptr<ov::Model> model,
 
     if (embed_result) {
         auto shape = embed_result->input(0).get_shape();
-        // If shape.size() is 3, then last axis should be the Vocab size.
+        // If shape.size() is 3, then last axis should contain the rank of embedding dimension.
         // But 1st and 2nd axes can mean different things.
         // 1st axis can represent the batch size, while 2nd - the number of embeddings,
         // or vice-versa (in chatglm)
         if (shape.size() == 3) {
+            OPENVINO_ASSERT(batch_dim <= 1, "Unexpected value of batch_dim: ", batch_dim, ", expected 0 or 1!");
             uint32_t num_embeds_dim = 1 - batch_dim;
-            if (shape[num_embeds_dim] > max_generation_token_len) {
+            OPENVINO_ASSERT(shape[num_embeds_dim] >= max_generation_token_len,
+                            "Number of output embeddings should be greater or equal to the slicing range!");
+            if (shape[num_embeds_dim] != max_generation_token_len) {
                 std::vector<int32_t> start_pos{
                     static_cast<int32_t>(batch_dim * (shape[num_embeds_dim] - max_generation_token_len)),
                     static_cast<int32_t>(num_embeds_dim * (shape[num_embeds_dim] - max_generation_token_len)),

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

@@ -858,7 +858,9 @@ void ov::npuw::LLMInferRequest::infer_generate(ov::SoPtr<ov::ITensor> input_ids,
 
     if (!m_generate_initialized) {
         LOG_DEBUG("Copy kv-cache from prefill to generate model.");
-        copy_kvcache();
+        if (kvcache_desc.num_stored_tokens > 0) {
+            copy_kvcache();
+        }
 
         LOG_DEBUG("Prepare inputs.");
         namespace uu = ov::npuw::util;