Ukraine has been using a patchwork of different weapons and calibers since the wars beginning. If we want to say all infantry weapons being sent are of the same caliber they have two old Soviet weapons and now NATO standard hardware. Grunts can carry a 7.62x39mm AK, a 5.56x45mm M4, or a 5.45x39mm AK. Ukrainian soldiers have been spotted carrying even older caliber weapons like the Mosin's 7.62x54mmR, 7.62x51mm NATO in FAL rifles, and I wouldn't be surprised if older equipment is in use. If this drags out long I'm guessing we will see the gradual standardization around 5.56 with NATO equipment as stores of Russian surplus runs out if it hasn't already and Ukraine is relying on captured ammunition for it's AK platform weapons.

On another note I am surprised the US hasn't supplied 5.56 chambered AK's, habits die hard in soldiers and I'm sure many would like to have an AK made reliably in the caliber being supplied.