How do I tell grad_method = "F" that I want a certain step size? I know that I can set the step size in Qnode but doing this sets the step size for the entire circuit. In this case I want to just set the step size for this particular class.
I.e., this is what your eventual use case would look like. You would have your RXX gate in some larger circuit (f in the classical analogue) with other tunable parameters. But, for the parameters associated with RXX, use a different shift.
The PennyLane dev team is working on supporting this, though!
Out of curiosity, is there a reason why you’re not using parameter-shift? There is more operator-to-operator tweaking available in this case currently. From the generator of the RXX gate, you can use this to obtain parameter frequencies. With this, all you’d add to your class is: