`gflownet/gflownet.py` - should not store `self.env` as well as `self.env_maker`

[Joseph Viviano]

currently, self.env is stored in the GFlowNet class, because other classes expect an env instance (to access various methods / attributes).

Since the gflownet now stores a class factory rather than a class instance, we should figure out another way to communicate with these other classes (instead of storing an env instance.