add slot record suuport for GpuPS

paddle/fluid/framework/channel.h

@@ -157,7 +157,19 @@ class ChannelObject {
     p.resize(finished);
     return finished;
   }
+  // read once only
+  size_t ReadOnce(std::vector<T>& p, size_t size) {  // NOLINT
+    if (size == 0) {
+      return 0;
+    }
+    std::unique_lock<std::mutex> lock(mutex_);
+    p.resize(size);
+    size_t finished = Read(size, &p[0], lock, true);
+    p.resize(finished);
+    Notify();
 
+    return finished;
+  }
   size_t ReadAll(std::vector<T>& p) {  // NOLINT
     p.clear();
     size_t finished = 0;
@@ -241,17 +253,21 @@ class ChannelObject {
     return !closed_;
   }
 
-  size_t Read(size_t n, T* p, std::unique_lock<std::mutex>& lock) {  // NOLINT
+  size_t Read(size_t n, T* p, std::unique_lock<std::mutex>& lock,  // NOLINT
+              bool once = false) {                                 // NOLINT
     size_t finished = 0;
     CHECK(n <= MaxCapacity() - reading_count_);
     reading_count_ += n;
     while (finished < n && WaitForRead(lock)) {
-      size_t m = std::min(n - finished, data_.size());
+      size_t m = (std::min)(n - finished, data_.size());
       for (size_t i = 0; i < m; i++) {
         p[finished++] = std::move(data_.front());
         data_.pop_front();
       }
       reading_count_ -= m;
+      if (once && m > 0) {
+        break;
+      }
     }
     reading_count_ -= n - finished;
     return finished;