Evaluation of input and output impedance of a RF amplifier
So, for instance for evaluating the output impedance, the input voltage Vin is shorted (so the input port is shorted). Is this the same procedure through which the output impedance of an RF amplifier is evaluated? Or is its input port closed on a 50Ohm load?

As user1850479 says in comments, we often characterize RF components using s-parameters rather than impedance or admittance parameters.
When evaluating the s-parameters of an n-port device we terminate all rf ports with the system characteristic impedance. In this case that would be 50 ohms.
In the case of measuring \$S_{11}\$ or \$S_{22}\$ (forward and reverse reflection parameters, from which the input and output impedance can be derived) of an active amplifier, due to the isolation characteristic of the amplifier, the termination of port 2 is unlikely to affect \$S_{11}\$ significantly and vice versa. In a passive device (a transformer, or an attenuator, for example), the terminations will more likely affect the reflection parameters.