No, it’s called unpacking (look it up, it’s common in python)
It’s shorthand for
W_grad = grad_fn(X_train,y_train,W,b)[0]
b_grad = grad_fn(X_train,y_train,W,b)[1]
It will only work if grad_fn(...) returns an iterable with two elements, otherwise it will fail.
solved Two variables with the same value in Python