update validation step to support pretraining

revise validation step to support pre-training

add code for embedding resize when pre-training

Update README.md

add length limit in bash file

add updates on hf version

update readme

update the link to pre-trained data

add links to pre-training data in readme

add missing argument for pre-training

minor changes in default value

add bash files to use the scripts and link to fully supervised models.

add descriptions in the beginning.

add instructions on daobtaining datasets.

add instructions on generating pre-training data and link to newshead dataset.