Save code of registered custom models (#15379)

* Allow dynamic modules to use relative imports

* Work for configs

* Fix last merge conflict

* Save code of registered custom objects

* Map strings to strings

* Fix test

* Add tokenizer

* Rework tests

* Tests

* Ignore fixtures py files for tests

* Tokenizer test + fix collection

* With full path

* Rework integration

* Fix typo

* Remove changes in conftest

* Test for tokenizers

* Add documentation

* Update docs/source/custom_models.mdx

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Add file structure and file content

* Add more doc

* Style

* Update docs/source/custom_models.mdx

Co-authored-by: Suraj Patil <surajp815@gmail.com>

* Address review comments

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Suraj Patil <surajp815@gmail.com>

utils/test_module/custom_tokenization_fast.py

@@ -0,0 +1,8 @@
+from transformers import BertTokenizerFast
+
+from .custom_tokenization import CustomTokenizer
+
+
+class CustomTokenizerFast(BertTokenizerFast):
+    slow_tokenizer_class = CustomTokenizer
+    pass