fix concat_mkldnn op. test=develop (#22692)

fix concat_mkldnn op when encounter extreame conditions.

paddle/fluid/operators/mkldnn/concat_mkldnn_op.cc

@@ -142,9 +142,12 @@ class ConcatMKLDNNOpKernel : public paddle::framework::OpKernel<T> {
         paddle::framework::ToMKLDNNDataType(multi_input[0]->type());
 
     ConcatPrimitiveFactory<T> prim_creator;
+    // If one of the multiple inputs of concat has an input size of 0, the
+    // actual size of the multi_input will change
     std::string key = platform::CreateKey(
         paddle::framework::vectorize<int>(multi_input[0]->dims()),
-        ctx.OutputName("Out"), dt, platform::ThreadIDasStr());
+        multi_input.size(), ctx.OutputName("Out"), dt,
+        platform::ThreadIDasStr());
 
     const std::string key_prim = key + "@concat_p";
     const std::string key_concat_pd = key + "@concat_pd";