Hi i have a simple idea but i dunno cant it be work or not.
Just connect the input to a diode follow by an resistor then follow
by a parallel capacitor and a LED.
When input 1, the diode will forward bias and starting recharge the capacitor and at the same time ramp up the LED. When the input change from 1 to 0 the diode stop conduct and the capacitor start to discharge and ramp down the LED. Adjust the resistor and capacitor value to adjust the ramp up and ramp down time.