AllReduceOp missing verification for missing operand

### Request description

AllReduceOp allows variadic operands, and we implicitly require at least 1 operand. This is enforced in MHLO [here](https://github.com/tensorflow/mlir-hlo/blob/745a8f527e254496fec45e399d52cab231915f25/mhlo/IR/hlo_ops.cc#L2556-L2558), but not documented nor enforced in StableHLO.