add export cpu_nums for supporting cpu environment (#3302)

* add export cpu_nums for supporting cpu environment

* add export cpu_nums for supporting cpu environment

PaddleNLP/language_representations_kit/ELMo/run.sh

+#if use gpu， you could specify gpu devices
 export CUDA_VISIBLE_DEVICES=0 
+#if use cpu, you could specify cpu nums
+export CPU_NUM=10
 python  train.py \
 --train_path='data/train/sentence_file_*'  \
 --test_path='data/dev/sentence_file_*'  \