MLM training for DeBERTa not supported: configuration class is missing
When I ran the example script run_mlm.py to fine tune the pretrained deberta model on a customized dataset, I got the following error. The same command worked for roberta-base.

The command:
python run_mlm.py --model_name_or_path 'microsoft/deberta-base' --train_file slogans/train.txt --validation_file slogans/test.txt --do_train --do_eval --per_device_train_batch_size 64 --per_device_eval_batch_size 64 --learning_rate 1e-3 --num_train_epochs 10 --output_dir /home/jovyan/share2/xiaolin/models/mlm/temp --save_steps 5000 --logging_steps 100

The terminal error:
Traceback (most recent call last):
  File "run_mlm.py", line 409, in <module>
    main()
  File "run_mlm.py", line 264, in main
    cache_dir=model_args.cache_dir,
  File "/home/jovyan/.local/lib/python3.7/site-packages/transformers/models/auto/modeling_auto.py", line 1093, in from_pretrained
    config.__class__, cls.__name__, ", ".join(c.__name__ for c in MODEL_FOR_MASKED_LM_MAPPING.keys())
ValueError: Unrecognized configuration class <class 'transformers.models.deberta.configuration_deberta.DebertaConfig'> for this kind of AutoModel: AutoModelForMaskedLM.
Model type should be one of LayoutLMConfig, DistilBertConfig, AlbertConfig, BartConfig, CamembertConfig, XLMRobertaConfig, LongformerConfig, RobertaConfig, SqueezeBertConfig, BertConfig, MobileBertConfig, FlaubertConfig, XLMConfig, ElectraConfig, ReformerConfig, FunnelConfig, MPNetConfig, TapasConfig. 
Looking at the [docs](https://huggingface.co/transformers/model_doc/deberta.html), it seems like there's currently no `DeBERTaForMaskedLM` defined. I will make a PR that adds this.