[s2s/distill] remove run_distiller.sh, fix xsum script (#8412)

examples/seq2seq/run_distiller.sh

-# Add parent directory to python path to access lightning_base.py
-export PYTHONPATH="../":"${PYTHONPATH}"
-
-python distillation.py \
---learning_rate=3e-4 \
---do_train \
---do_predict \
---fp16 \
---val_check_interval 0.1 \
-"$@"

examples/seq2seq/train_distilbart_xsum.sh

@@ -2,6 +2,7 @@
 export PYTHONPATH="../":"${PYTHONPATH}"
 python distillation.py \
   --teacher facebook/bart-large-xsum --data_dir xsum \
+  --tokenizer_name facebook/bart-large-xsum \
   --student_decoder_layers 6 --student_encoder_layers 12 \
   --freeze_encoder --freeze_embeds \
   --learning_rate=3e-4 \