code: public boolean isPlaceHolder(String varName) {
          Preconditions.checkState(variables.containsKey(varName), "No variable present in SameDiff instance with name \"%s\"", varName);
          return variables.get(varName).getVariable().isPlaceHolder();
      }
docs: Returns true if this vertex id is a place holder variable or not<br>
  A place holder variable is one where the array shape(s) are currently known and can't yet be calculated
  
  @param varName the vertex id to test
  @return True if the variable is a placeholder, false otherwise