' if we employ any other process that is not adiabatic, say an isochoric process, to take the system from one temperature to another, we shall need a series of reservoirs in the temperature range $T_1$ to $T_2$ to ensure that at each stage the process is quasi static '

This means if we want to have a reversible process between two temperatures, then we should remove all irreversibilities. One of them is heat transferring due to definite temperature difference. So, if we want a reversible process, then we need a series of reservoirs in the temperature range $T_1$ to $T_2$ and temperature difference between each reservoir and next one should be $dT$.

' if we employ any other process that is not adiabatic, say an isochoric process, to take the system from one temperature to another, we shall need a series of reservoirs in the temperature range $T_1$ to $T_2$ to ensure that at each stage the process is quasi static '

This means if we want to have a reversible process between two temperatures, then we should remove all irreversibilities. One of them is heat transferring due to definite temperature difference. So, if we want a reversible process, then we need a series of reservoirs in the temperature range $T_1$ to $T_2$ and temperature difference between each reservoir and next one should be $\mathrm dT$.