XGBoost: default params when GPU is enabled

[exalate-issue-sync]

From investigation of AutoML results between {{rel-yau-6}} and {{rel-yau-11}}, we notice a degradation of performance in XGBoost training.
See the discussion at [https://h2oai.slack.com/archives/C0E0ADTM1/p1576683431088900?thread_ts=1576604011.073300&cid=C0E0ADTM1|https://h2oai.slack.com/archives/C0E0ADTM1/p1576683431088900?thread_ts=1576604011.073300&cid=C0E0ADTM1]

Suggestions:

• disable parallel training of CV models for XGBoost if GPU is enabled: method {{XGBoost#nModelsInParallel}}.
• use {{updater= grow_gpu_hist}} on GPU.

[exalate-issue-sync]

Jan Sterba commented: fixed, the rafactor to using tree_method for GPU had the side-effect of suing gpu_predictor withch probably made CV models slower

[h2o-ops]

JIRA Issue Migration Info

Jira Issue: PUBDEV-7164
Assignee: Jan Sterba
Reporter: Sebastien Poirier
State: Resolved
Fix Version: 3.28.0.2
Attachments: N/A
Development PRs: Available

Linked PRs from JIRA

#4186
#4257