add dygraph amp support for transformer (#5187)

* add dygraph amp support for transformer

* according params use_amp to decide use dygraph amp

PaddleNLP/benchmark/transformer/dygraph/train.py

@@ -126,14 +126,26 @@ def do_train(args):
             train_reader_cost = time.time() - batch_start
             (src_word, trg_word, lbl_word) = input_data
 
-            logits = transformer(src_word=src_word, trg_word=trg_word)
-
-            sum_cost, avg_cost, token_num = criterion(logits, lbl_word)
-
-            avg_cost.backward()
-
-            optimizer.step()
-            optimizer.clear_grad()
+            if args.use_amp:
+                scaler = paddle.amp.GradScaler(
+                    init_loss_scaling=args.scale_loss)
+                with paddle.amp.auto_cast():
+                    logits = transformer(src_word=src_word, trg_word=trg_word)
+                    sum_cost, avg_cost, token_num = criterion(logits, lbl_word)
+
+                scaled = scaler.scale(avg_cost)  # scale the loss
+                scaled.backward()  # do backward
+
+                scaler.minimize(optimizer, scaled)  # update parameters
+                optimizer.clear_grad()
+            else:
+                logits = transformer(src_word=src_word, trg_word=trg_word)
+                sum_cost, avg_cost, token_num = criterion(logits, lbl_word)
+
+                avg_cost.backward()
+
+                optimizer.step()
+                optimizer.clear_grad()
 
             tokens_per_cards = token_num.numpy()