log for fallback added

paddle/fluid/framework/operator.cc

@@ -637,6 +637,7 @@ void OperatorWithKernel::RunImpl(const Scope& scope,
   // workaround for missing MKLDNN kernel when FLAGS_use_mkldnn env var is set
   if (kernel_iter == kernels.end() &&
       expected_kernel_key.library_type_ == LibraryType::kMKLDNN) {
+    VLOG(3) << "missing MKLDNN kernel: fallbacking to PLAIN one";
     expected_kernel_key.library_type_ = LibraryType::kPlain;
     expected_kernel_key.data_layout_ = DataLayout::kAnyLayout;
     kernel_iter = kernels.find(expected_kernel_key);