I have a few questions about variable frequency drive closed-loop torque control:

1) Where to set the torque, does the VFD has corresponding parameters?
2) Does the VFD drive's multi-function analog input port only can control the frequency?
3) When torque control is the slave torque machine control mode, at this time the torque setting is base on the host torque, then how to get it?

I read a lot of information, the current torque is determined by the current loop, then is it means the current loop is multi-function analog input?

You need to choose a vector control variable frequency drive.

The VFD has following control modes: speed control, torque control, minimum speed torque control, maximum speed torque control, speed torque plus mode.
The analog input of the VFD drive can be set by parameters for speed control and torque given, the PID regulator feedback etc.
For master and slave mode, generally the master drive adopts speed mode, the slave drive adopts torque mode, send the master drives speed loop output (torque input) to the slave drive as as the given torque by some ways(such as analog outputs, network, etc.)

1: There are not so many variable frequency drive can be set as torque control. Set the parameters according to the VFD brand
2: Set the torque control analog input interface can control torque
3: did not get your means for the third point

What is your control object?
Speed? add a rotary encoder and measure speed through the VFD drive PG card
Pressure? install a pressure transmitter detects pressure
Detect signal as a negative feedback and send to the variable frequency drive or the controller

The so-called closed-loop control is commonly used in the PID algorithm
There are two modes, one is the use VFD PID function
Another way is to use a PLC or smart regulator PID control
The first way's advantage is to save money, but not intuitive with friendly interface
The second method is just to make up the shortcomings of the first way, but you need to pay more.

1 determines the closed-loop object, select the closed-loop detection devices, such as pressure sensors, temperature sensors. . .
2 Select the right AC drives
3 look at the drive's user manual carefully
4 Hardware connection: closed loop testing equipment (typically 0 - 5V,0 - 10V,4 - 20mA) to connect the ac drive
5 AC drive parameters setting: the frequency given, given mode, PID parameters given.
Also, if the closed-loop parameters is a lot it's necessary to add auxiliary equipment, like PLC, HMI touch screen