[JVM-Packages] Remove synchronized in java predict function.

[trivialfis]

We have made the predict function thread safe in 1.4.

Follow up on #6021 . Related #5951 .

[trivialfis]

Hmm, it's not the predict function not being thread-safe, but rather the jvm iterator.

[Craigacp]

I think removing the synchronized keyword should be sufficient (at least for our use case in Tribuo). What do you mean by the JVM iterator isn't thread safe?

[trivialfis]

It looks like XGBoost is failing to construct DMatrix from java iterator for some reasons.

[Craigacp]

If you can point me at a test or some example code I can run that down.

Edit: Oh I see it now. I don't think I could see the other CI runs before, but the link seems to work now. So it's failing in the middle of Spark when trying to make a DMatrix?

I'm not sure how this change would affect that.

[trivialfis]

I will take a deeper dive into jvm prediction after #6819 is done.

[gangpeng]

@trivialfis
Hi,
We would like to remove the synchronized keyword from predict method from xgboost4j for only project. We only construct DMatrix from float[] in production, the code path looks thread-safe to me. Is the problem you mentioned only related to iterable?
Thanks!