Can not place an obj with fixed_base on other obj using "OnTop"

[dexin-wang]

Describe the bug
Can not place an obj with fixed_base on other obj using "OnTop".
Reason: During the OnTop operation, obj1 is moved to a position above obj2 with z_offset=5cm. However, the fixed_base property prevents obj1 from falling automatically, causing the subsequent Touching check to fail, ultimately leading to repeated attempts and eventual failure.

To Reproduce

import omnigibson as og
import torch
from omnigibson import object_states

cfg = dict()

# Define scene
cfg["scene"] = {
    "type": "Scene",
    "floor_plane_visible": True,
}

# Define objects
cfg["objects"] = [
    {
        "type": "DatasetObject",
        "name": "fridge",
        "category": "fridge",
        "model": "dszchb",
        "position": [0, 5, 10],
        # "bounding_box": [1.065, 1.149, 1.528],
        "fixed_base": True
    },
    {
        "type": "DatasetObject",
        "name": "ground",
        "category": "floors",
        "model": "2w2nau",
        "position": [10, 10, 0.1],
        "bounding_box": [5, 5, 0.1],
        "fixed_base": True,
        "visible": True,
        # "visual_only": True
    },
    {
        "type": "LightObject",
        "name": "brilliant_light",
        "light_type": "Sphere",
        "intensity": 50000,
        "radius": 0.1,
        "position": [3.0, 3.0, 4.0],
    },
]

# Create the environment
env = og.Environment(cfg)
env.step([])

fridge = env.scene.object_registry("name", 'fridge')
ground = env.scene.object_registry("name", 'ground')
fridge.states[object_states.OnTop].set_value(ground, True)

for t in range(10000):
    obs, rew, terminated, truncated, info = env.step(torch.empty(0))
    
og.clear()

Expected behavior
Obj1 (fridge) is placed on top of obj2 (ground).

Screenshots
no need.

Desktop (please complete the following information):

• OS: [e.g. Ubuntu 20.04]
• Isaac Sim Version [e.g. 4.1.0]
• OmniGibson Version [e.g. 1.1.0]

Additional context
A simple improvement solution:
(1) set z_offset = 0 if obj1 is fixed_base in omnigibson/utils/object_state_utils.py/def sample_kinematics(),
(2) do not check Touching for obj1 with fixed_base in omnigibson/object_states/on_top.py/def _get_value().