fix megatron bert convert state dict naming
Author: itszyin
Pull request: yes

# What does this PR do?

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Fixes some key of state_dict changing error in Megatron-Bert convert script. In checkpoint v3, MegatronLM seems to change `attention` -> `self_attention`, and HF has updated this key at GPT convert script[#12007](https://github.com/huggingface/transformers/pull/12007/files) while the BERT is still not.
 
```
layers.0.input_layernorm.weight
layers.0.input_layernorm.bias
layers.0.self_attention.query_key_value.weight
layers.0.self_attention.query_key_value.bias
layers.0.self_attention.dense.weight
layers.0.self_attention.dense.bias
layers.0.post_attention_layernorm.weight
layers.0.post_attention_layernorm.bias
layers.0.mlp.dense_h_to_4h.weight
layers.0.mlp.dense_h_to_4h.bias
layers.0.mlp.dense_4h_to_h.weight
layers.0.mlp.dense_4h_to_h.bias
```


## Who can review?

@LysandreJik 

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15820). All of your documentation changes will be reflected on that endpoint.

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Merging