Add low-latency kernel PCIe usage flag (#195)

* Add low-latency kernel usage flag

* Update comments

csrc/deep_ep.hpp

@@ -71,6 +71,10 @@ private:
     volatile int* moe_recv_rdma_counter = nullptr;
     int* moe_recv_rdma_counter_mapped = nullptr;
 
+    // Host-side low-latency kernels' usages
+    volatile int* low_latency_usage_flag = nullptr;
+    int* low_latency_usage_flag_mapped = nullptr;
+
 private:
     void move_fifo_slots(int num_slots = 1);
 
@@ -132,6 +136,8 @@ public:
                       const torch::Tensor& combined_rdma_head, const torch::Tensor& combined_nvl_head,
                       const Config& config, std::optional<EventHandle>& previous_event, bool async, bool allocate_on_comm_stream);
 
+    uint64_t get_low_latency_usage_flag() const;
+
     void clean_low_latency_buffer(int num_max_dispatch_tokens_per_rank, int hidden, int num_experts);
 
     std::tuple<torch::Tensor, std::optional<torch::Tensor>, torch::Tensor, torch::Tensor, torch::Tensor, std::optional<EventHandle>, std::optional<std::function<void()>>>