[Perf] avoid duplicate mem_get_info() call in get_current_memory_usage (#33064)

Signed-off-by: Paco Xu <paco.xu@daocloud.io>

vllm/platforms/rocm.py

@@ -561,7 +561,8 @@ class RocmPlatform(Platform):
         cls, device: torch.types.Device | None = None
     ) -> float:
         torch.cuda.reset_peak_memory_stats(device)
-        return torch.cuda.mem_get_info(device)[1] - torch.cuda.mem_get_info(device)[0]
+        free_mem, total_mem = torch.cuda.mem_get_info(device)
+        return total_mem - free_mem
 
     @classmethod
     def get_device_communicator_cls(cls) -> str: