Implement AsyncTextIteratorStreamer for asynchronous streaming (#34931)

* Add AsyncTextIteratorStreamer class

* export AsyncTextIteratorStreamer

* export AsyncTextIteratorStreamer

* improve docs

* missing import

* missing import

* doc example fix

* doc example output fix

* add pytest-asyncio

* first attempt at tests

* missing import

* add pytest-asyncio

* fallback to wait_for and raise TimeoutError on timeout

* check for TimeoutError

* autodoc

* reorder imports

* fix style

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Co-authored-by: Arthur Zucker <arthur.zucker@gmail.com>
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

docs/source/en/internal/generation_utils.md

@@ -352,6 +352,8 @@ A [`Constraint`] can be used to force the generation to include specific tokens
 
 [[autodoc]] TextIteratorStreamer
 
+[[autodoc]] AsyncTextIteratorStreamer
+
 ## Caches
 
 [[autodoc]] Cache