Add norm_by_time for warpctc op in padding mode. (#17580)

paddle/fluid/operators/warpctc_op.h

@@ -267,6 +267,11 @@ class WarpCTCGradKernel : public framework::OpKernel<T> {
       size_t num_sequences = warpctc_grad->dims()[1];
       size_t seq_width = warpctc_grad->dims()[2];
 
+      auto* logits_length = ctx.Input<framework::Tensor>("LogitsLength");
+      framework::Tensor logits_length_cpu;
+      framework::TensorCopy(*logits_length, platform::CPUPlace(),
+                            &logits_length_cpu);
+
       LoDTensor logits_grad_with_lod;
       auto logits_grad_dims =
           framework::make_ddim({static_cast<int64_t>(max_seq_length),
@@ -289,10 +294,14 @@ class WarpCTCGradKernel : public framework::OpKernel<T> {
       const T* loss_grad_data = loss_grad_cpu.data<T>();
       for (size_t i = 0; i < max_seq_length; ++i) {
         for (size_t j = 0; j < num_sequences; ++j) {
+          T scale = 1.0;
+          if (norm_by_times) {
+            scale = 1.0 / static_cast<T>(logits_length_cpu.data<int64_t>()[j]);
+          }
           for (size_t k = 0; k < seq_width; ++k) {
             size_t idx = i * (num_sequences * seq_width) + j * seq_width + k;
             scaled_logits_data[idx] =
-                logits_grad_cpu_data[idx] * loss_grad_data[j];
+                logits_grad_cpu_data[idx] * loss_grad_data[j] * scale;
           }
         }
       }