Returns the local seeds an operation should use given an op-specific seed.
Given operation-specific seed,
op_seed, this helper function returns two
seeds derived from graph-level and op-level seeds. Many random operations
internally use the two seeds to allow user to change the seed globally for a
graph, or for only specific operations.
For details on how the graph-level seed interacts with op seeds, see
A tuple of two integers that should be used for the local seed of this operation.