* Enable running objectives with 0 GPU. * Enable 0 GPU for objectives. * Add doc for GPU objectives. * Fix some objectives defaulted to running on all GPUs.
The documentation of xgboost is generated with recommonmark and sphinx. You can build it locally by typing "make html" in this folder. Checkout https://recommonmark.readthedocs.org for guide on how to write markdown with extensions used in this doc, such as math formulas and table of content.