[Feature request] Support batched conditional generation from GPT-2 
# 🚀 Feature request

Support batched conditional generation from GPT-2 

## Motivation

Currently the [method](https://github.com/huggingface/transformers/blob/9ca485734aea269961d63a040ff194365d151fd1/src/transformers/modeling_utils.py#L802) to generate text from GPT-2 conditioned on an input sequence only supports either 1) a single input at a time, or 2) a batch of inputs where the conditioning input sequence is the same length. It would be great (for efficiency) if this method could be updated to support a batch with conditional inputs of varying length, done by ignoring padding in the input_ids. 

## Your contribution

Unlikely to have time to code this, but will submit a PR if I do. 

Also see: https://github.com/huggingface/transformers/issues/3021<|||||>This is known to not work at the moment with `generate()`. I have to think a bit about the cleanest way to implement it :-) Code suggestions are very welcome! 

<|||||>Very interested in this! Came here from #3021 (many hours after wondering why my batch generation was not working...)<|||||>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.
