fix: rebind domain randomization modders after a hard reset

[VihaanAgarwal]

What this does

Fixes #426 (🐛 Bug)

When an environment uses the mujoco renderer, a hard reset frees the old MjSim and builds a new one (_destroy_sim then _initialize_sim in MujocoEnv.reset). DomainRandomizationWrapper.reset called save_default_domain() before rebinding its modders to the new sim, so the modders still pointed at the freed sim. Reading a model parameter from it then raised:

AttributeError: 'MjSim' object has no attribute 'model'

Reproducer from the issue:

import robosuite as suite
from robosuite.wrappers import DomainRandomizationWrapper

env = suite.make("Door", robots="Panda", has_renderer=True, has_offscreen_renderer=False, use_camera_obs=False)
env = DomainRandomizationWrapper(env)
env.reset()

The update_sim loop that rebinds the modders already existed, it just ran after save_default_domain(). Moving it ahead of the save so the modders read from the live sim is enough to fix it. This is also what was suggested in the issue thread.

How it was tested

Added test_domain_randomization_hard_reset in tests/test_environments/test_domain_randomization.py. It wraps a Lift env with the mujoco renderer and resets it across several hard resets. The test fails on master with the AttributeError above and passes with this change. randomize_color is left off in the test because TextureModder requires mujoco==3.1.1.

black, isort and flake8 are clean on the changed files.

How to checkout & try? (for the reviewer)

python -m pytest tests/test_environments/test_domain_randomization.py