[docs] fix xref to `PreTrainedModel.generate`
This PR partially resolves the issue raised in https://github.com/huggingface/transformers/issues/9202

I spent quite some time to try to figure out how to get sphinx to figure out the inheritance so that it could cross-reference inherited methods, but it can't even handle mixins it seems. i.e. it can't resolve: `transformers.PreTrainedModel.generate` 

So one has to explicitly specify the fully qualified original method for xref to work :(

Bottom line - if currently you want a cross-reference link to `transformers.PreTrainedModel.generate` or `T5ForConditionalGeneration.generate` to work, you have to use `~transformers.generation_utils.GenerationMixin.generate`

So I did:
```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|transformers\.\w+\.generate|transformers.generation_utils.GenerationMixin.generate|g' {} \;
```

then also added the same for `transformers.PreTrainedModel.(greedy_search|sample|beam_search|beam_sample|group_beam_search)`
```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's#transformers.PreTrainedModel.(greedy_search|sample|beam_search|beam_sample|group_beam_search)#transformers.generation_utils.GenerationMixin.$1#g' {} \;
```

Fixes: https://github.com/huggingface/transformers/issues/9202

@sgugger 
Would it be a good idea to make an Good second Issue for someone to research whether sphinx could be configured to resolve inherited and mixin methods when it does cross-referencing? It'd be much more intuitive for doc writing if we could have the sub-classed method named and also not needing to hide the sub-class via `~` prefix.

I did spent quite some time trying to find one and also experimenting with different configs but it doesn't mean it doesn't exist.<|||||>Sure, we can try that. I didn't try for very long but didn't manage it on my side either, for what it's worth.<|||||>https://github.com/huggingface/transformers/issues/11069<|||||>https://github.com/huggingface/transformers/issues/11069 had many fishermen but nothing came out of it. So I guess I will merge it and close https://github.com/huggingface/transformers/issues/11069<|||||>Fine by me!