config.attention_head_size for structured pruning out-of-box
# 🚀 Feature request

## Motivation

for structured pruning like `fastformers`, https://github.com/microsoft/fastformers#pruning-models ,

we should modify the source code of transformers for `attention_head_size.

for example,

1. configuration_bert.py
https://github.com/microsoft/fastformers/blob/main/src/transformers/configuration_bert.py#L128

2. modeling_bert.py
https://github.com/microsoft/fastformers/blob/main/src/transformers/modeling_bert.py#L192
https://github.com/microsoft/fastformers/blob/main/src/transformers/modeling_bert.py#L263

is it possible to set `attention_head_size` from outside(config.json) ?

## Your contribution


@dsindex  FYI, I am working on creating a PR including this feature for the effort of #8083 .<|||||>@ykim362 great!  closing issue here.