!12027 Insert a placeholder at the front and back of embedding table to support ps cache sparse mode

From: @gaoyong10
Reviewed-by: @cristoval,@limingqi107
Signed-off-by: @limingqi107

mindspore/ccsrc/ps/ps_cache/embedding_hash_map.h

@@ -51,6 +51,11 @@ class EmbeddingHashMap {
         graph_running_index_pos_(0),
         expired_element_full_(false) {
     hash_map_elements_.resize(hash_capacity);
+    // In multi-device mode, embedding table are distributed on different devices by ID interval,
+    // and IDs outside the range of local device will use the front and back positions of the table,
+    // the positions are reserved for this.
+    hash_map_elements_.front().set_step(SIZE_MAX);
+    hash_map_elements_.back().set_step(SIZE_MAX);
     graph_running_index_ = std::make_unique<int[]>(hash_capacity);
   }
   virtual ~EmbeddingHashMap() = default;