Fix bugs on training Parametric Models

[BirkhoffG]

At this point, relax.generate_cf_explanations will train the ParametricCFModule no matter what. However, if this ParametricCFModule has already been trained, we should not repeat this process.

For example,

cfnet = CounterNet()
# we train cfnet here
cfnet.train(data_module)

# but relax will train `cfnet` again here
exp = relax.generate_cf_explanations(cfnet, data_module)

Delivery

Fix bug here:

jax-relax/relax/explain.py

Line 193 in 02a72a7

if isinstance(cf_module, ParametricCFModule):

1. Check ParametricCFModule.is_trained is True or not under this if statement.
2. Write test cases

You might also need to fix bugs in some of the methods inherent ParametricCFModule.