How to pretrain BERT whole word masking (wwm) model?
## 🚀 Feature

Code to pretrain BERT whole word masking (wwm) model

## Motivation

WWM offers better performance, but the current codebase doesn't seem to support this feature.

## Additional context

Related i #1352 
Any ideas on whether this will be included sooner or later?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
