Fix custom generate from local directory (#38916)

Fix custom generate from local directory:
1. Create parent dirs before copying files (custom_generate dir)
2. Correctly copy relative imports to the submodule file.
3. Update docs.

docs/source/en/generation_strategies.md

@@ -468,9 +468,17 @@ def generate(model, input_ids, generation_config=None, left_padding=None, **kwar
 Follow the recommended practices below to ensure your custom decoding method works as expected.
 - Feel free to reuse the logic for validation and input preparation in the original [`~GenerationMixin.generate`].
 - Pin the `transformers` version in the requirements if you use any private method/attribute in `model`.
-- You can add other files in the `custom_generate` folder, and use relative imports.
 - Consider adding model validation, input validation, or even a separate test file to help users sanity-check your code in their environment.
 
+Your custom `generate` method can relative import code from the `custom_generate` folder. For example, if you have a `utils.py` file, you can import it like this:
+
+```py
+from .utils import some_function
+```
+
+Only relative imports from the same-level `custom_generate` folder are supported. Parent/sibling folder imports are not valid. The `custom_generate` argument also works locally with any directory that contains a `custom_generate` structure. This is the recommended workflow for developing your custom decoding method.
+
+
 #### requirements.txt
 
 You can optionally specify additional Python requirements in a `requirements.txt` file inside the `custom_generate` folder. These are checked at runtime and an exception will be thrown if they're missing, nudging users to update their environment accordingly.