message: [XLA:GPU] Improve all-reduce-related error message.
  This error message is wrong; all-reduce *is* now implemented, but it's only
  implemented in unnested contexts, i.e. not inside fusions, maps, etc.
diff: @@ -655,8 +655,8 @@ Status IrEmitter::HandleFft(HloInstruction* fft) {
  }
  Status IrEmitter::HandleAllReduce(HloInstruction* crs) {
  - // TODO(b/33011107): Support cross replica sum on GPU.
  - return Unimplemented("AllReduce is not implemented on GPU.");
  + return Unimplemented(
  + "AllReduce cannot be nested inside of fusion, map, etc.");
  }
  Status IrEmitter::HandleParameter(HloInstruction* parameter) {
  