[Bugfix] Fix torch.compile x LoRA for PyTorch 2.8 (#20823)

Signed-off-by: rzou <zou3519@gmail.com>

Author: zou3519

vllm/lora/layers.py

@@ -240,17 +240,19 @@ def set_lora(
     def forward(self, x: torch.Tensor) -> torch.Tensor:
         added_tokens_mask = torch.where(x > self.base_layer.org_vocab_size - 1,
                                         1, 0)
-        embeddings_indices = torch.narrow(
-            self.punica_wrapper._embeddings_indices, 1, 0, x.size(0))
 
-        indices = embeddings_indices[1]
+        # NB: Don't use torch.narrow here. torch.narrow triggers some
+        # Dynamic Shape specialization in torch.compile
+        num_tokens = x.shape[0]
+        indices_1 = self.punica_wrapper._embeddings_indices[1][:num_tokens]
+        indices_0 = self.punica_wrapper._embeddings_indices[0][:num_tokens]
+
         full_lora_a_embeddings = F.embedding(
-            x + indices,
+            x + indices_1,
             self.lora_a_stacked_2d,
         )
-        indices = embeddings_indices[0]
         full_output = self.base_layer.forward(x +
-                                              (indices * added_tokens_mask))
+                                              (indices_0 * added_tokens_mask))
 
         full_output_org = full_output
         if full_output.ndim == 3: