Jiaming Yuan 24241ed6e3
[EM] Compress dense ellpack. (#10821)
This helps reduce the memory copying needed for dense data. In addition, it helps reduce memory usage even if external memory is not used.

- Decouple the number of symbols needed in the compressor with the number of features when the data is dense.
- Remove the fetch call in the `at_end_` iteration.
- Reduce synchronization and kernel launches by using the `uvector` and ctx.
2024-09-20 18:20:56 +08:00
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