ValueError: Expected dict, got None.

Hi! Thanks for this implementation!

I am trying to use this implementation but I am running into the error in the title of this issue. Here is what I am working with:

```python
import jax
import jax.numpy as jnp
import jax.scipy.stats as stats

from typing import Callable, Tuple
import haiku as hk
from jax.random import PRNGKey, split

import optax
import matplotlib.pyplot as plt

from optax_swag import swag

def nll(apply_fn: Callable):

def _nll(params, batch: Tuple[jax.Array, jax.Array]) -> float:
x, y = batch
out = apply_fn(params, x)
ll = stats.norm.logpdf(out, y)
return - ll.sum()

return _nll

def generate_data():
x = jnp.linspace(0, 10, 25).reshape(-1, 1)
y = jnp.sin(0.4 * x) + 3
return x, y

def make_small_mlp():

relu = jax.nn.relu
def small_mlp(x):

mlp = hk.Sequential([
hk.Linear(50),
relu,
hk.Linear(50),
relu,
hk.Linear(50),
relu,
hk.Linear(1)])

return mlp(x)

return hk.transform(small_mlp)

def train_model(params, model_apply, data, opt_init, opt_update, epochs, loss_fn):

loss_fn = nll(model_apply)
x, y = data
opt_state = opt_init(params)
print(opt_state)

@jax.jit
def train_one_epoch(params, opt_state):

nll_val, grad = jax.value_and_grad(loss_fn)(params, (x,y))
updates, opt_state = opt_update(grad, opt_state)
params = optax.apply_updates(params, updates)
return params, opt_state, nll_val

for i in range(epochs):
params, opt_state, nll_val = train_one_epoch(params, opt_state)

print(f"STEP {i} | NLL: {nll_val}")
preds = model_apply(params, x)

plt.plot(x, y)
plt.plot(x, preds)
plt.show()

return params

model_init_key, _, _, _, _ = split(PRNGKey(123), 5)
x,y = generate_data()

mlp = make_small_mlp()
params = mlp.init(model_init_key, x[0])

model_apply = lambda params, x: mlp.apply(params, None, x)
opt_init, opt_update = optax.chain(optax.adam(1e-3), swag(5, 5))
params = train_model(params, model_apply, (x,y), opt_init, opt_update, 500, nll)
# 'ValueError: Expected dict, got None.'
```

Any ideas of what I may doing wrong? Thanks so much!