Base score for custom multi-output objects is hard-coded at 0.5

**EDIT:** sorry, first version of this issue was wrong, I've now edited it.

When using multi-output objectives with multi-output trees, the intercept / base score / first tree seems to start at 0.5 regardless of what the labels are.

In this example, I am passing a multi-output label in which the second column is the negative of the first one:
```python
import numpy as np, xgboost as xgb
mtcars = np.array([[21,6,160,110,3.9,2.62,16.46,0,1,4,4],
[21,6,160,110,3.9,2.875,17.02,0,1,4,4],
[22.8,4,108,93,3.85,2.32,18.61,1,1,4,1],
[21.4,6,258,110,3.08,3.215,19.44,1,0,3,1],
[18.7,8,360,175,3.15,3.44,17.02,0,0,3,2],
[18.1,6,225,105,2.76,3.46,20.22,1,0,3,1],
[14.3,8,360,245,3.21,3.57,15.84,0,0,3,4],
[24.4,4,146.7,62,3.69,3.19,20,1,0,4,2],
[22.8,4,140.8,95,3.92,3.15,22.9,1,0,4,2],
[19.2,6,167.6,123,3.92,3.44,18.3,1,0,4,4],
[17.8,6,167.6,123,3.92,3.44,18.9,1,0,4,4],
[16.4,8,275.8,180,3.07,4.07,17.4,0,0,3,3],
[17.3,8,275.8,180,3.07,3.73,17.6,0,0,3,3],
[15.2,8,275.8,180,3.07,3.78,18,0,0,3,3],
[10.4,8,472,205,2.93,5.25,17.98,0,0,3,4],
[10.4,8,460,215,3,5.424,17.82,0,0,3,4],
[14.7,8,440,230,3.23,5.345,17.42,0,0,3,4],
[32.4,4,78.7,66,4.08,2.2,19.47,1,1,4,1],
[30.4,4,75.7,52,4.93,1.615,18.52,1,1,4,2],
[33.9,4,71.1,65,4.22,1.835,19.9,1,1,4,1],
[21.5,4,120.1,97,3.7,2.465,20.01,1,0,3,1],
[15.5,8,318,150,2.76,3.52,16.87,0,0,3,2],
[15.2,8,304,150,3.15,3.435,17.3,0,0,3,2],
[13.3,8,350,245,3.73,3.84,15.41,0,0,3,4],
[19.2,8,400,175,3.08,3.845,17.05,0,0,3,2],
[27.3,4,79,66,4.08,1.935,18.9,1,1,4,1],
[26,4,120.3,91,4.43,2.14,16.7,0,1,5,2],
[30.4,4,95.1,113,3.77,1.513,16.9,1,1,5,2],
[15.8,8,351,264,4.22,3.17,14.5,0,1,5,4],
[19.7,6,145,175,3.62,2.77,15.5,0,1,5,6],
[15,8,301,335,3.54,3.57,14.6,0,1,5,8],
[21.4,4,121,109,4.11,2.78,18.6,1,1,4,2]])
y = mtcars[:, 0]
X = mtcars[:, 1:]

def rmse_obj(predt, dtrain):
print(predt[:5])
raise ValueError()
y = dtrain.get_label().reshape(predt.shape)
grad = (y - predt).reshape(-1)
hess = np.ones_like(grad)
return grad,hess

dm = xgb.DMatrix(data=X, label=np.c_[y, -y])
model = xgb.train(
dtrain=dm,
params={
"tree_method": "hist",
"multi_strategy": "multi_output_tree",
},
num_boost_round=3,
obj=rmse_obj
)
```
```
[[0.5 0.5]
[0.5 0.5]
[0.5 0.5]
[0.5 0.5]
[0.5 0.5]]
```

Both have the same score of 0.5, which doesn't look like it'd be a better choice than zero, for example.