[Bug] Issues when running continuous domain example

Hello,

I've been following the tutorial steps found [here](https://google.github.io/dopamine/docs/) which work pretty well for discrete domains. However, when it comes to the example using SAC I get the following error:

`Traceback (most recent call last):
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/runpy.py", line 197, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/runpy.py", line 87, in _run_code
exec(code, run_globals)
File "/home/usr/Documents/Inria/dopamine/dopamine/continuous_domains/train.py", line 55, in
app.run(main)
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/absl/app.py", line 308, in run
_run_main(main, args)
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/absl/app.py", line 254, in _run_main
sys.exit(main(argv))
File "/home/usr/Documents/Inria/dopamine/dopamine/continuous_domains/train.py", line 49, in main
runner = run_experiment.create_continuous_runner(base_dir)
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/gin/config.py", line 1605, in gin_wrapper
utils.augment_exception_message_and_reraise(e, err_str)
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/gin/utils.py", line 41, in augment_exception_message_and_reraise
raise proxy.with_traceback(exception.__traceback__) from None
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/gin/config.py", line 1582, in gin_wrapper
return fn(*new_args, **new_kwargs)
File "/home/usr/Documents/Inria/dopamine/dopamine/continuous_domains/run_experiment.py", line 92, in create_continuous_runner
return ContinuousRunner(base_dir, create_continuous_agent)
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/gin/config.py", line 1605, in gin_wrapper
utils.augment_exception_message_and_reraise(e, err_str)
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/gin/utils.py", line 41, in augment_exception_message_and_reraise
raise proxy.with_traceback(exception.__traceback__) from None
File "/home/usr/miniconda3/envs/dopamine/lib/python3.9/site-packages/gin/config.py", line 1582, in gin_wrapper
return fn(*new_args, **new_kwargs)
File "/home/usr/Documents/Inria/dopamine/dopamine/continuous_domains/run_experiment.py", line 164, in __init__
self._initialize_checkpointer_and_maybe_resume(checkpoint_file_prefix)
File "/home/usr/Documents/Inria/dopamine/dopamine/discrete_domains/run_experiment.py", line 303, in _initialize_checkpointer_and_maybe_resume
if self._agent.unbundle(
File "/home/usr/Documents/Inria/dopamine/dopamine/jax/agents/sac/sac_agent.py", line 615, in unbundle
self.network_params = bundle_dictionary['network_params']
KeyError: 'network_params'
In call to configurable 'ContinuousRunner' ()
In call to configurable 'create_continuous_runner' ()`

I'm sorry for not providing further details since I just started using the library :).