[FLAX] glue training example refactor (#13815)

* refactor run_flax_glue.py

* updated readme

* rm unused import and args typo fix

* refactor

* make consistent arg name across task

* has_tensorboard check

* argparse -> argument dataclasses

* refactor according to review

* fix

examples/flax/text-classification/README.md

@@ -33,15 +33,16 @@ export TASK_NAME=mrpc
 python run_flax_glue.py \
   --model_name_or_path bert-base-cased \
   --task_name ${TASK_NAME} \
-  --max_length 128 \
+  --max_seq_length 128 \
   --learning_rate 2e-5 \
   --num_train_epochs 3 \
   --per_device_train_batch_size 4 \
+  --eval_steps 100 \
   --output_dir ./$TASK_NAME/ \
   --push_to_hub
 ```
 
-where task name can be one of cola, mnli, mnli-mm, mrpc, qnli, qqp, rte, sst2, stsb, wnli.
+where task name can be one of cola, mnli, mnli_mismatched, mnli_matched, mrpc, qnli, qqp, rte, sst2, stsb, wnli.
 
 Using the command above, the script will train for 3 epochs and run eval after each epoch. 
 Metrics and hyperparameters are stored in Tensorflow event files in `--output_dir`.