The handling of unknown ops will be tightened in a subsequent change. All sparse_tensor ops should implement BufferizableOpInterface, otherwise, they are treated as "unknown" and additional buffer allocs/copies may be inserted around them.
Depends On: D142728
The ToIndicesOp (as well as following ToXXX) returns a field of the sparse tensor, I am not sure whether it should be considered as an alias to the sparse tensor or not.