Make the attention_head_size in distilbert an object attribute
# What does this PR do?

It simply moves the attention_head_size in the distilbert model to be an object attribute. This is necessary if you want to use the Distilbert model in the nn_pruning library. It will also benefit anyone who ever needs to access the attention_head_size attribute from an instance of a Distilbert model. This change is consistent with other transformer models in this library (see BERT https://github.com/huggingface/transformers/blob/main/src/transformers/models/bert/modeling_bert.py#L253 or BART https://github.com/huggingface/transformers/blob/main/src/transformers/models/bart/modeling_bart.py#L157)
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for your PR. Could you just run `make style` on your branch to fix the quality issue?

Hi @sgugger, thanks for the quick approval. Just fixed the code style<|||||>Thanks again for your contribution!