Ensure same results with ModelBuilder.fit random_seed

[wd60622]

Brought up in #958, there are different results depending on the way on the random_seed is set.

# Doesn't work
model.fit(random_seed=42)
model.fit(..., random_seed=np.random.default_rng(42))
# Does work
model = ModelBuilder(..., sampler_config={"random_seed": 42})
model.fit(...)
# Doesn't work because of np.random.Generator is not serializable
model = ModelBuilder(..., sampler_config={"random_seed": np.random.default_rng(42)})
model.fit(...)

Current workaround: Use an int random_seed parameter in sampler_config at initialization.

See the #958 for more details.