[Docs] Improve PyTorch, Flax generate API (#15988)

* Move generate docs

* up

* Update docs/source/_toctree.yml

* correct

* correct some stuff

* correct tests

* more fixes

* finish generate

* add to doc stest

* finish

* finalize

* add warning to generate method

docs/source/main_classes/model.mdx

@@ -86,14 +86,6 @@ Due to Pytorch design, this functionality is only available for floating dtypes.
     - push_to_hub
     - all
 
-## Generation
-
-[[autodoc]] generation_utils.GenerationMixin
-
-[[autodoc]] generation_tf_utils.TFGenerationMixin
-
-[[autodoc]] generation_flax_utils.FlaxGenerationMixin
-
 ## Pushing to the Hub
 
 [[autodoc]] file_utils.PushToHubMixin