[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/_toctree.yml

@@ -114,6 +114,8 @@
       title: Logging
     - local: main_classes/model
       title: Models
+    - local: main_classes/text_generation
+      title: Text Generation
     - local: main_classes/onnx
       title: ONNX
     - local: main_classes/optimizer_schedules