message: Update node_def.proto comments
  The device field had outdated comments.
  Note: We could consider adding tpu as an example here, e.g. "gpu" | "cpu" | "tpu".  Thoughts?
diff: @@ -35,7 +35,7 @@ message NodeDef {
  // CONSTRAINT ::= ("job:" JOB_NAME)
  // | ("replica:" [1-9][0-9]*)
  // | ("task:" [1-9][0-9]*)
  - // | ( ("gpu" | "cpu") ":" ([1-9][0-9]* | "*") )
  + // | ("device:" ("gpu" | "cpu") ":" ([1-9][0-9]* | "*") )
  //
  // Valid values for this string include:
  // * "/job:worker/replica:0/task:1/device:GPU:3" (full specification)
  