Fix `get_relative_import_files` for module import
Author: MayDomine
Pull request: yes

# What does this PR do?

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Fixes # (issue)
`get_relative_import_files` from dynamic_module_utils.py will crash when the `object_file` is a module instead a python file. This issue will make custom `save_function` for `save_pretrained` function of `PretrainedModel` crash. 
This PR fix this function to make it possible to handle module import.

https://github.com/huggingface/transformers/blob/bc161d5d06ba3b40eaca99ce42734b4f1fecfaa2/src/transformers/dynamic_module_utils.py#L127C2-L135C63

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