Pre-training language model with Translation Language Modelling (TLM) objective 
### Feature request

For large-scale pre-training MLM objective is heavily used. eg: BERT (Devlin et al., 2018), XLM-R (Connaue et al.,2019). The resources available from huggingface is to train a language model (LM) using MLM or CLM objectives. (ie. https://github.com/huggingface/notebooks/blob/main/examples/language_modeling.ipynb)

To recreate XLM (Lample and Connaue, 2019) I wish to pre-train my own language model using both MLM and TLM objectives. Please advice on how to do this using the huggingface transformers.

Thank you.

### Motivation

MLM + TLM is a common pre-training objective during language modelling training. Therefore for further improvement to these models, we need to first train using MLM + TLM objectives.  In addition to this, if I need to customize the masking to be done for noun terms only, or verb terms only, please let me know whether this is allowed.

### Your contribution

I would appreciate if the pre-training on TLM objective is also provided.
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>If this is supported in the codebase please provide the steps or refer to an available resource.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.