Add Changelog for Training Operator v1.8.0-rc.0

[andreyvelich]

I added changelog for Training Operator v1.8.0-rc.0.
Thanks everyone for your contribution to Training Operator 🎉

Also, I updated the changelog generation script.
We might need to update the release doc to follow similar process as for Katib: https://github.com/kubeflow/katib/tree/master/docs/release

/hold for review
/assign @kubeflow/wg-training-leads @deepanker13 @tariq-hasan @afritzler @champon1020 @shruti2522 @kellyaa @qzoscar @champon1020 @PeterWrighten @garymm @lowang-bh @double12gzh @rpemsel @franciscojavierarceo @jdcfd @shashank-iitbhu @daniel-hutao @kuizhiqing @testwill @Syulin7 @rpemsel @kubeflow/release-team

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[coveralls]

Pull Request Test Coverage Report for Build 9018726340

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 35.383%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	6	80.67%

Totals
Change from base Build 8997849275:	-0.03%
Covered Lines:	4374
Relevant Lines:	12362

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💛 - Coveralls