[TPU] Temporary fix vmem oom for long model len by reducing page size (#20278)

Signed-off-by: Chenyaaang <chenyangli@google.com>

vllm/v1/attention/backends/pallas.py

@@ -86,6 +86,12 @@ class PallasAttentionBackend(AttentionBackend):
     # spill less likely. Meanwhile we make sure the page size is in [16, 256].
     @staticmethod
     def get_page_size(vllm_config: VllmConfig) -> int:
+        # TODO: This is a temporary fix for vmem OOM.
+        # For long model length, we use 16 page-size to avoid too much
+        # VMEM spill. A more robust solution should be implemented to
+        # handle VREG spills.
+        if vllm_config.model_config.max_model_len > 8192:
+            return 16
         page_size = next_power_of_2(
             vllm_config.model_config.max_model_len) // 16
         if page_size <= 16: