Add a toggle option for pluggable Grappler optimizer. (Refactored from #47635.)
Add a toggle option for pluggable Grappler optimizer. (Refactored from #47635.)

Context:
[1] Grappler C API RFC
https://github.com/tensorflow/community/blob/master/rfcs/20201027-modular-tensorflow-graph-c-api.md
[2] Pluggable Grappler pass registration PR #47635.
PiperOrigin-RevId: 363803765
Change-Id: I14b46889fc87bf7d64215dd54788398cae5a846f

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