added failing NaN test
fixed operator_unary_increment so gradient returns NaN
added failing NaN test
fixed operator_unary_decrement so gradients return NaN
added failing NaN test
fixed operator_multiplication so gradients return NaN for NaN inputs
fixed NaN test for operator_multiply_equal
added failing NaN test
fixed operator_subtraction so gradient returns NaN for NaN input
fixed NaN test for operator_minus_equal
added failing NaN test
fixed operator_division so gradients return NaN for NaN inputs
fixed NaN test for operator_divide_equal
added failing NaN test
fixed agrad/rev/operator_addition so gradient returns NaN for NaN inputs
updated NaN test for operator_plus_eq