[CUDA] Make CudaEvent work with multi-device (#2614)

* Set current device when creating cuda event

* Separate cuda events by device

* Avoid race condition in pool

mlx/backend/cuda/eval.cpp

@@ -15,9 +15,10 @@ bool is_available() {
 }
 
 void new_stream(Stream s) {
-  // Force initalization of CUDA by creating an event, so the CUDA runtime and
-  // our CUDA event pool get destroyed last.
-  cu::CudaEvent(cudaEventDefault);
+  // Force initalization of CUDA, so CUDA runtime get destroyed at last.
+  cudaFree(nullptr);
+  // Make sure CUDA event pool get destroyed after device and stream.
+  cu::CudaEvent::init_pool();
   // Ensure the static stream objects get created.
   cu::get_command_encoder(s);
 }