Add `BloomForQuestionAnswering`
# What does this PR do?

This PR adds the class `BloomForQuestionAnswering`, insipred from [`GPTJForQuestionAnswering`](https://github.com/huggingface/transformers/blob/4c962d5e790d06c142af35aad165c74c0bcf861a/src/transformers/models/gptj/modeling_gptj.py#L1024), as the community asked for the release of this class (See the discussion here: https://huggingface.co/bigscience/bloom/discussions/46#633b35f21fd49ee0b64e29d2) 

cc @sgugger @ydshieh @LysandreJik  @ArthurZucker 
_The documentation is not available anymore as the PR was closed or merged._<|||||>I have just fixed your suggestions @sgugger  💪 Gently pinging you here, and let me know if you need me to open a PR for GPTJ too ;) !<|||||>Thanks a lot!!