bert: add conversion script for BERT Token Dropping TF2 checkpoints (#17142)
* bert: add conversion script for BERT Token Dropping TF2 checkpoints
* bert: rename conversion script for BERT Token Dropping checkpoints
* bert: fix flake errors in BERT Token Dropping conversion script
* bert: make doc-builder happy!!1!11
* bert: fix pytorch_dump_path of BERT Token Dropping conversion script