Why is \$R_{eq}=0\$ ?and because the \$\tau=\infty\$.so in fact inductor is always in the Transient state and be charged?
Is it because that there is no resistor in the Charging path of inductor,so the \$R_{eq}=0\$ ?


It's because at \$t=0^+\$, SW1 is closed and SW2 is open. That puts a short in parallel with the inductor.


  \$i_L(t)=i_L(\infty)+[i_L(0)-i_L(\infty)]e^{-\frac{t}{\tau}}=i_L(\infty)(1-e^{-\frac{t}{\tau}})+i_L(0)\$,


and \$\tau=\infty\$,so is the inductor always be charged?

Yes, as the circuit stands during this time, there is no loss mechanism. Therefore the inductor does not discharge.