chore: Update the use of MultiWorkerMirroredStrategy in TF

[terrytangyuan]

The current implementation is experimental and deprecated.

_CollectiveAllReduceStrategyExperimental.__init__ (from tensorflow.python.distribute.collective_all_reduce_strategy) is deprecated and will be removed in a future version.
Instructions for updating:
use distribute.MultiWorkerMirroredStrategy instead

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: terrytangyuan

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [terrytangyuan]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[coveralls]

Pull Request Test Coverage Report for Build 3810119636

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.03%) to 39.231%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	77.18%
pkg/controller.v1/pytorch/pytorchjob_controller.go	9	59.52%

Totals
Change from base Build 3808317272:	0.03%
Covered Lines:	2663
Relevant Lines:	6788

---

💛 - Coveralls

[johnugeorge]

/lgtm