On the second integral moment of $L$-functions

Assume that the generalized Ramanujan conjecture holds on the automorphic $L$-function $L(s, \pi)$ on $\GL_d$ over $\mathbb{Q}$ with $d\geq 3$, we can obtain a small log-saving non-trivial bound on the second integral moment of $L(1/2+it, \pi)$. Specifically the bound $$\int_{T}^{2T}\Big|L\big(\frac{1}{2}+it, \pi\big)\Big |^2 \dd t\ll_{\pi} \frac{T^{\frac{d}{2}}}{\log^{\eta_d}T}$$ holds for a small constant $\eta_d>0$.