code: public SDVariable placeHolder(String name, org.nd4j.linalg.api.buffer.DataType dataType, long...shape) {
          SDVariable ret = new SDVariable(name, VariableType.PLACEHOLDER, this, shape, dataType, null);
          variables.put(name, Variable.builder().name(name).variable(ret).build());
          return ret;
      }
docs: Create a a placeholder variable. Placeholders are variables that expect an array to be provided during training
  and inference.<br>
  For example, the SDVariables for your input/features and labels should be placeholders.<br>
  See also: {@link VariableType}
  
  @param name     the name of the variable
  @param dataType Data type of the new placeholder
  @param shape    the shape of the variable if any
  @return SDVariable placeholder