Fix deprecation warning

[timothygebhard]

Fixes the deprecation warning documented in #12.

---

Sanity check: Running this before the change:

import normflows as nf
import torch

torch.manual_seed(42)

flow = nf.NormalizingFlow(
    nf.distributions.DiagGaussian(1, trainable=False),
    [
        nf.flows.AutoregressiveRationalQuadraticSpline(1, 1, 1),
        nf.flows.LULinearPermute(1)
    ]
)

with torch.no_grad():
    samples_flow, _ = flow.sample(4)

print(samples_flow)

gives:

tensor([[0.4528],
        [0.6410],
        [0.5200],
        [0.5567]])

After the change, the output stays the same.