Thunder's vjp doesn't compute the result for variadic arguments

[IvanYashchuk]

🐛 Bug

import torch
import thunder

def f(*args):
    a, b = args
    c = a + b
    return c

def ff(*args):
    return thunder.core.transforms.value_and_grad(f)(*args)

a = torch.ones(3)
b = torch.ones(3)
cff = thunder.jit(ff)
print(cff(a, b)) # Returns (tensor([2., 2., 2.]), (None, None))

There should be ones instead of Nones.

It's probably happening because "unpacking" operations are ignored in

lightning-thunder/thunder/core/transforms.py

Line 2638 in f7b2e15

def backward_pass(forward_env, trace, init_cotangents):