MegatronBertForMaskedLM
Author: kaushalshetty
Pull request: no

## Environment info
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- `transformers` version: 4.12.5
- Platform: Linux
- Python version: 3.6
- PyTorch version (GPU?): 1.10.0+cu102
- Tensorflow version (GPU?):2.6
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
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Models:

- ALBERT, BERT, XLM, DeBERTa, DeBERTa-v2, ELECTRA, MobileBert, SqueezeBert: @LysandreJik
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- Wav2Vec2, HuBERT, SpeechEncoderDecoder, UniSpeech, UniSpeechSAT, SEW, SEW-D, Speech2Text: @patrickvonplaten, @anton-l

If the model isn't in the list, ping @LysandreJik who will redirect you to the correct contributor.

Library:

- Benchmarks: @patrickvonplaten
- Deepspeed: @stas00
- Ray/raytune: @richardliaw, @amogkam
- Text generation: @patrickvonplaten @narsil
- Tokenizers: @SaulLu
- Trainer: @sgugger
- Pipelines: @Narsil
- Speech: @patrickvonplaten, @anton-l
- Vision: @NielsRogge, @sgugger

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@LysandreJik @stas00 
## Information

Model I am using MegatronBERT from https://huggingface.co/nvidia/megatron-bert-uncased-345m. The model loads correctly in the following way : 
```
from transformers import BertTokenizer, MegatronBertModel
model = MegatronBertModel.from_pretrained("megatron_model_here")
```

but throws a RuntimeError for size mismatch while using MegatronBertForMaskedLM 

## To reproduce
`model = MegatronBertForMaskedLM.from_pretrained("megatron_model_here")
`
      
Error : 
```
RuntimeError: Error(s) in loading state_dict for MegatronBertForMaskedLM:
	size mismatch for cls.predictions.bias: copying a param with shape torch.Size([30592]) from checkpoint, the shape in current model is torch.Size([29056])
```

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## Expected behavior

MaskedLM model loads properly


Comments:
I was able to reproduce the issue.

Please try: https://github.com/huggingface/transformers/pull/16639


@stas00  Thanks!!! Works like a charm now.