[Python] How to reuse the .gradient_index.page cache files for ExtMemQuantileDMatrix?

I have a custom data iterator that I pass into the constructor for `ExtMemQuantileDMatrix`, which when called produces huge cache files with names of the form `cache-*.gradient_index.page`. Is there a way to reuse these cache files between runs, so that the same `ExtMemQuantileDMatrix` does not need to be recreated from scratch?

Start with the Python ExtMemQuantileDMatrix constructor and trace how the custom data iterator creates cache-*.gradient_index.page files. Determine whether those files can safely be reused between runs; done should be a documented reuse path or a clearly documented limitation, with coverage for repeated construction if the project has an applicable test location.

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