* Add FP16 option in save_combine_op

* Fix casting issue

We should improve our test isolation before raising the
level of parallelism.

This reverts commit 0446220e.

Reason:

Rerun failed test hides flaky test.
Flaky test can be bugs, for example, race condition.
Test shouldn't be flaky, if a test is flaky, it should be fixed.

* CI: rerun failed tests.

* fix check style error.