[s2s] dynamic batch size with --max_tokens_per_batch (#7030)

examples/seq2seq/dynamic_bs_example.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+export PYTHONPATH="../":"${PYTHONPATH}"
+export WANDB_PROJECT=dmar
+export MAX_LEN=128
+export m=sshleifer/student_marian_en_ro_6_1
+python finetune.py \
+  --learning_rate=3e-4 \
+  --do_train \
+  --fp16 \
+  --data_dir wmt_en_ro \
+  --max_source_length $MAX_LEN --max_target_length $MAX_LEN --val_max_target_length $MAX_LEN --test_max_target_length $MAX_LEN \
+  --freeze_encoder --freeze_embeds \
+  --train_batch_size=48 --eval_batch_size=64 \
+  --tokenizer_name $m --model_name_or_path $m --num_train_epochs=1 \
+  --warmup_steps 500 --logger_name wandb --gpus 1 \
+  --fp16_opt_level=O1 --task translation \
+  "$@"