So the reason why torque converter is needed is because it disconnects engine from transmission. It has also extra benefit of multiplying torque but that is not 100% necessary. From what I have seen all automatics have some sort of constant input which is always connected to the shaft. Would it be possible to design an automatic transmission which wouldn't have constant input and therefore wouldn't need torque converter?
That is not why torque converters are used in automatic transmissions.
They are used because in addition to declutching the engine from the transmission at idle, they furnish torque multiplication, in essence giving a 3-speed transmission a fourth "gear" that slips into and out of engagement without jerks and furnishes a smooth range of equivalent "gear ratios" without requiring inputs from the driver or the drive management computer.
It is possible to design automatic transmissions without torque converters but they require servo-controlled clutches for starting and stopping, and you must also add at least one additional fixed gear ratio to the transmission to make up for the loss of same by elimination of the torque converter.
The original automatics did not have torque converters ; they mostly had fluid couplings . Chrysler in particular , had more than a half dozen different ones in the various cars. They were more like "gear changers" with an automatic clutch ; some still included a clutch pedal. Performance was very poor by todays standards. And there were overdrives that would make one shift . The GM Hydromatic was the first good automatic ( 1939 Olds and 1940 Cadillac as I remember) . It had a fluid coupling that had limited torque converter function - I forget the percentage. It could shift hard enough to burn rubber in the first 2 shifts ( with the V-8 engines).