[CUDA] Reduce use of managed memory (#2725)

* Use async cuda malloc managed with cuda 13

* add pool threshold

* refactor for regular cuda malloc

* load eval gpu for cuda

* remove use of cuda pool, use cuda free async

* fix

* fix

* fix

* fix

* fix + comment

mlx/backend/cuda/cudnn_utils.cpp

@@ -132,14 +132,18 @@ bool prepare_cudnn_plan(
     void** data_ptrs,
     F&& execute) {
   int workspace_size = plan.getWorkspaceSize();
-  array workspace(
-      workspace_size > 0 ? allocator::malloc(workspace_size)
-                         : allocator::Buffer(nullptr),
-      {workspace_size},
-      uint8);
+  void* workspace_ptr = nullptr;
+  if (workspace_size > 0) {
+    array workspace(
+        cu::malloc_async(workspace_size, encoder.stream()),
+        {workspace_size},
+        uint8);
+    encoder.add_temporary(workspace);
+    workspace_ptr = gpu_ptr<void>(workspace);
+  }
 
   auto args = cudnn_frontend::VariantPackBuilder()
-                  .setWorkspacePointer(workspace.data<void>())
+                  .setWorkspacePointer(workspace_ptr)
                   .setDataPointers(num_args, data_ptrs)
                   .setUids(num_args, uids)
                   .build();
@@ -151,7 +155,6 @@ bool prepare_cudnn_plan(
     return false;
   }
 
-  encoder.add_temporary(workspace);
   return true;
 }