Darlington Transistor \$ A_I , A_V \$
how can we find the current and voltage gain?


They are both zero.


For the gains we need to consider the small signal situation. We can't consider the big signal situation because the \$h_{FE}\$ of transistor Q2 will be so wayward that we could easily be out by a factor of ten i.e. it becomes very much device and temperature dependent.
So, I'm assuming that the input is a small voltage AC source biased at a DC voltage that delivers an output DC voltage of around 50% Vcc. I'm also assuming that Vcc is 12 volts because gain (in the absence of a Q2 emitter resistor) will be very much dependent on supply voltage.
So, if there is 6 volts DC at the collector of Q2 then there will be 6 mA flowing through RC2. This means 2 mA into RL and 4 mA through Q2. At ambient temperatures, \$r_E\$ will be \$\frac{26\text{ mV}}{4\text{ mA}}\$ or about 6 Ω.
The Q2 stage voltage gain will therefore be approximately: -
$$\dfrac{RL||RC2}{r_E} \approx 125 $$
Q1 is a voltage follower and has a gain of 1 hence, small signal voltage gain of the whole circuit is about 125.
The current gain of Q1 is defined by what transistor it is. Typically, for a modern transistor this can vary from 100 to 400 so let's say 200 as a midway point.
It's a the same story for Q2 so, overall current gain is typically around 200² = 40,000.