fix position bias related logic after prune heads in T5 model
# What does this PR do?

A follow up pr after #17968

If the attention layer `self.has_relative_attention_bias == False`, then the position bias shape will be wrong, the head number should be the original model (before prune heads) head number `self.n_heads + len(self.pruned_heads)`

## Who can review?

- t5: @patrickvonplaten @patil-suraj
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19097). All of your documentation changes will be reflected on that endpoint.<|||||>Maybe of interest to @ArthurZucker :)<|||||>Hey, before diving a bit deeper, sorry for the long delay, and thanks for the PR. 
Would you mind adding a test? 🤗 I can take care of it otherwise! <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Closing in favor of #20106. Thanks for your contribution