

TITLE
Inconsistency between GPTNeo and GPT2 config classes

COMMENTS
6

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BODY
The config classes for GPTNeo and GPT2 have a bunch of differences that are seemingly unnecessary. This makes it harder for downstream users to write code that depends on accessing these attributes. See below:

![image](https://user-images.githubusercontent.com/54557097/122113739-bbf92300-cddf-11eb-9ac6-a0ea1a30055b.png)
It seems that max_position_embeddings, hidden_size, num_layers, num_heads, intermediate_size, resid_dropout, embed_dropout, and attention_dropout should be renamed for sonsistency with the GPT2 config class.

### Who can help
@LysandreJik @patil-suraj 