Change DistributedVarOp to be a non-tuple class
Change DistributedVarOp to be a non-tuple class

Some functions takes both update_op and (old_value, new_value) and uses istuple
to dispatch them. We need var.assign().op not to be a tuple.

Cleaning up DistributedVarOp turns out more complicated than anticipated.
There're a lot of mysterious test failures. Let's not block fixing variable
assign bug on them.

PiperOrigin-RevId: 302828354
Change-Id: I924cb5ee117b2c67f4161df6eb695a7c54bd7cfe

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