`test_vjp_correctness` fails with ops that return tensors that do not require grads.

@kshitij12345 is already working on this.

Since Apr 29, 2024.

## 🐛 Bug

As per title. To reproduce, one could uncomment these tests in these tests in https://github.com/Lightning-AI/lightning-thunder/pull/118 to get:

```python
thunder/tests/test_grad.py:423: in test_vjp_correctness
result = run_snippet(
thunder/tests/framework.py:483: in run_snippet
raise ex
thunder/tests/framework.py:475: in run_snippet
snippet(*args, **kwargs)
thunder/tests/test_grad.py:394: in snippet_vjp_correctness
check_vjp(func, *args, executor=executor)
thunder/tests/test_grad.py:304: in check_vjp
_, J_star_v = executor.make_callable_legacy(vjp(f), disable_torch_autograd_support=True)(primals, v)
thunder/common.py:783: in _fn
trc_or_result = trace(compile_data=cd)(processed_function, *args, **kwargs)
thunder/core/interpreter.py:1298: in fn_
return fn(*args, **kwargs)
thunder/common.py:534: in _trace
result = fn(*proxyargs, **proxykwargs)
thunder/core/transforms.py:3629: in _vjp
result, vjp_result = vjp_call(flat_args, cotangents, trace=trace)
thunder/core/transforms.py:3603: in vjp_call_metafunc
result, env = augmented_forward_pass(*primals, trace=trace, **kwargs)
thunder/core/transforms.py:3414: in augmented_forward_pass
result, env = eval_trace(
thunder/core/transforms.py:1693: in eval_trace
prim_func = symbol_mapper(symbol)
thunder/core/transforms.py:3338: in vjp_symbol_mapper
vjp_impl, backward_fn = make_aug_forward_and_backward(symbol)
thunder/core/vjp_utils.py:99: in make_aug_forward_and_backward
backward_bsyms = utils.find_producer_symbols(joint_trace, flat_bw_outputs, tree_flatten(bw_inputs)[0])
thunder/core/utils.py:1062: in find_producer_symbols
if arg_name not in map(lambda x: x.name, stop_proxies) and arg_name not in seen:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

x = None

> if arg_name not in map(lambda x: x.name, stop_proxies) and arg_name not in seen:
E AttributeError: 'NoneType' object has no attribute 'name'

thunder/core/utils.py:1062: AttributeError
```