DPR model examples / notebook / pipeline
# 🚀 Feature request

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     Please provide a link to the paper and code in case they exist. -->
An notebook or own script in the examples folders would be pretty nice for the new DPR model, implemented by @lhoestq 
Maybe an implementation to the pipeline would be cool too

## Motivation

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     related to a problem? e.g., I'm always frustrated when [...]. If this is related
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Such an notebook or script would make it easier to train an custom dpr model

## Your contribution

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I did not read the paper and had no closer look to the code, but with some advice I could try to start an PR

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
