message: ParseNodeName fix.
  ParseNodeName was skipping ops that started with an underscore, leading to warnings that input of an op was undefined and stopping grappler optimizations from being run on the graph.
diff: @@ -132,7 +132,7 @@ string ParseNodeName(const string& name, int* position) {
  strings::Scanner scan(name);
  scan.ZeroOrOneLiteral("^")
  .RestartCapture()
  - .One(strings::Scanner::LETTER_DIGIT_DOT)
  + .One(strings::Scanner::LETTER_DIGIT_DOT_UNDERSCORE)
  .Any(strings::Scanner::LETTER_DIGIT_DASH_DOT_SLASH_UNDERSCORE);
  StringPiece capture;
  StringPiece remaining;
  