!15946 Fix bug of all reduce fusion.

From: @jojobugfree
Reviewed-by: @kisnwang,@jjfeing
Signed-off-by: @jjfeing

mindspore/ccsrc/backend/session/session_basic.cc

@@ -2237,10 +2237,12 @@ void PreProcessOnSplitIndex(const KernelGraphPtr &graph, vector<uint32_t> *split
   uint32_t split_index_num = (*split_index)[split_index_len - 1];
   // obtain graph output tensor num
   auto grads_count = GetBpropGraphGradsCount(graph);
-  if (split_index_num == 0 || split_index_num >= grads_count) {
-    MS_LOG(EXCEPTION) << "invalid AllReduce split index " << split_index_num << " and grads count " << grads_count;
-  }
-  if (split_index_num < grads_count - 1) {
+  if (split_index_num >= grads_count) {
+    MS_LOG(WARNING) << "Invalid all_reduce_fusion_config:" << *split_index << " total grads count:" << grads_count
+                    << ". AllReduces are fused into one AllReduce.";
+    split_index->clear();
+    split_index->push_back(grads_count);
+  } else if (split_index_num < grads_count - 1) {
     split_index->push_back(grads_count - 1);
   }
 }