keep the output of linear tree-based model as probability estimates

[will945945945]

The outputs of a tree-based model should be in the range [0, 1]^{# of label},
which is corresponding to the probability estimates.
Moreover, if we want to use sparse matrix to store the prediction values of a tree model,
the value ``-inf'' will be a trouble issue.

What does this PR do?

1. modify the initial value of predictions from -inf to 0
2. replace the description decision values'' to probability estimates.''

Test CLI & API (bash tests/autotest.sh)

Test APIs used by main.py.

• Test Pass
  • (Copy and paste the last outputted line here.)
• Not Applicable (i.e., the PR does not include API changes.)

Check API Document

If any new APIs are added, please check if the description of the APIs is added to API document.

• API document is updated (linear, nn)
• Not Applicable (i.e., the PR does not include API changes.)

Test quickstart & API (bash tests/docs/test_changed_document.sh)

If any APIs in quickstarts or tutorials are modified, please run this test to check if the current examples can run correctly after the modified APIs are released.