[Fix] PagedKVCache fetching compute stream when copy stream is needed (#16714)

This PR fixes an issue in PagedKVCache, where a compute stream will
always be fetched. For backends like WebGPU, the `GetCurrentStream`
function is not implemented, which leads to an error when fetching
the compute stream.

Author: MasterJH5574

src/runtime/relax_vm/paged_kv_cache.cc

@@ -439,12 +439,12 @@ class PagedAttentionKVCacheObj : public AttentionKVCacheObj {
       free_page_ids_.push_back(page_id);
     }
 
-    // The compute stream is the default stream.
     // If the device is CUDA/ROCm, we create a standalone copy stream, in
     // purpose to hide the latency of auxiliary stream copy.
-    compute_stream_ = DeviceAPI::Get(device)->GetCurrentStream(device);
     if (device.device_type == DLDeviceType::kDLCUDA ||
         device.device_type == DLDeviceType::kDLROCM) {
+      // The compute stream is the default stream.
+      compute_stream_ = DeviceAPI::Get(device)->GetCurrentStream(device);
       copy_stream_ = DeviceAPI::Get(device)->CreateStream(device);
     }
   }