XGBoost 2.0.0 breaks tests about hummingbird HOT 8 OPEN

Since it seems they likely changed something internally (now it appears there is a one-node tree rather than a three-node tree), this will take a bit more time. Let's not block the release on this for now.

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Before, I believe they were defaulting to objective="rank:pairwise" but that doesn't seem to fix this.
Also, we'll need an upstream fix from onnxmltools for the 1 onnx XBG error.

Started troubleshooting at #733

Pinning this for now.

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onnxmltools was updated but let's wait for their release, then do ours

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With the onnxmltools patch, we now get some new errors:
==== 19 failed, 516 passed, 105 skipped, 115 warnings in 294.32s (0:04:54) ====

Some of them are related to the ndcg change

FAILED ....Check failed: label_is_valid: Relevance degress 
must be lesser than or equal to 31 when the exponential NDCG gain function is used. 
Set `ndcg_exp_gain` to false to use custom DCG gain.

but many are just outright wrong: Mismatched elements: 100 / 100 (100%) ...

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The problem is related to convert_sklearn_xgb_regressor's

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With xgb==1.7.6, for get_dump() we have '0:[f0<0.84472543] yes=1,no=2,missing=1\n\t1:leaf=-0.0600000024\n\t2:leaf=0.100000009\n',

whereas with xgb==2.0.0 we have '0:leaf=2.36637643e-09\n' from get_dump()

Then at:

the values for TreeParameters(lefts, rights, features, thresholds, values) are incorrect

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TODO: look at onnx/onnxmltools#597 more closely

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