Add methods to PreTrainedModel to use PyTorch's BetterTransformer (#21259)

* fix mess

* better documentation

* typo

* fix doc

* update

* add test

* fix test

* more tests

* Update src/transformers/modeling_utils.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* move to utils

* Apply suggestions from code review

Co-authored-by: Michael Benayoun <mickbenayoun@gmail.com>

* nit

---------

Co-authored-by: younesbelkada <younesbelkada@gmail.com>
Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Michael Benayoun <mickbenayoun@gmail.com>

src/transformers/testing_utils.py

@@ -65,6 +65,7 @@ from .utils import (
     is_librosa_available,
     is_natten_available,
     is_onnx_available,
+    is_optimum_available,
     is_pandas_available,
     is_phonemizer_available,
     is_pyctcdecode_available,
@@ -693,6 +694,13 @@ def require_bitsandbytes(test_case):
     return unittest.skipUnless(is_bitsandbytes_available(), "test requires bnb")(test_case)
 
 
+def require_optimum(test_case):
+    """
+    Decorator for optimum dependency
+    """
+    return unittest.skipUnless(is_optimum_available(), "test requires optimum")(test_case)
+
+
 def require_phonemizer(test_case):
     """
     Decorator marking a test that requires phonemizer