Fix fetch_tf_model download path handling for newer Keras versions

[rashidrao-pk]

Summary

This PR fixes fetch_tf_model when used with newer versions of Keras.

Recent versions of Keras no longer accept a full path passed through the fname argument of tf.keras.utils.get_file. Instead, the filename must be provided via fname, while the destination directory is specified using cache_dir.

This change updates fetch_tf_model accordingly, preserving the existing download location while resolving the get_file path error raised by newer Keras releases.

Changes

• Pass only the filename to tf.keras.utils.get_file.
• Specify the download directory using cache_dir.
• Add a regression test verifying that fetch_tf_model calls get_file with the expected arguments.

Testing

• Added a mocked regression test for fetch_tf_model.
• Verified that the new test passes locally.
• Confirmed that the original ValueError reported in issue alibi-detect: utils: fetch: "Paths are no longer accepted as the fname argument." #936 is resolved.

Closes #936

[CLAassistant]

All committers have signed the CLA.