message: Update node_def.proto
  Clarifying comments for valid device string in NodeDef, as discussed in PR
  Notes:
  1. The device string is as emitted by: tensorflow/python/framework/device.py to_string() function.
  2. I notice our regex convention does not use '+' (e.g. XX* instead of X+).
diff: @@ -35,7 +35,7 @@ message NodeDef {
  // CONSTRAINT ::= ("job:" JOB_NAME)
  // | ("replica:" [1-9][0-9]*)
  // | ("task:" [1-9][0-9]*)
  - // | ("device:" ("gpu" | "cpu") ":" ([1-9][0-9]* | "*") )
  + // | ("device:" [A-Za-z]* ":" ([1-9][0-9]* | "*") )
  //
  // Valid values for this string include:
  // * "/job:worker/replica:0/task:1/device:GPU:3" (full specification)
  