Add Pooler to DistilBERT
# 🚀 Feature request
Hi, I'd like to add a Pooler class to the DistilBERT model, whose interface is similar to BertPooler [here](https://github.com/huggingface/transformers/blob/7da995c00c025c4180c7fb0357256b7f83d342ef/src/transformers/models/bert/modeling_bert.py#L610)

## Motivation
I was using your DistilBERT model and discovered that I needed a pooler, so I wrote my own class. Thought I would add it to the repo in case others would like it.

## Your contribution
If this is something you're interested in, I can submit a PR
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