The assets data need to be packaged

## Bugs

When I tested the Isaac tutorial with elegantRL (use this [code](https://github.com/AI4Finance-Foundation/ElegantRL/blob/IsaacGym-Single-Process/demo_IsaacGym.py) the author mentioned in [this issue](https://github.com/AI4Finance-Foundation/ElegantRL/issues/169)). I found that **neither the config files nor the asset files can be loaded** without modifying the source code of ElegantRL. The error is shown as follows

### config files error
```python
Traceback (most recent call last):
File "/home/ubuntu/Github/Knowledge-Universe/Robotics/Roadmap-for-robot-science/rofunc/lfd/rl/envs/curi_env.py", line 96, in
demo(task)
File "/home/ubuntu/Github/Knowledge-Universe/Robotics/Roadmap-for-robot-science/rofunc/lfd/rl/envs/curi_env.py", line 45, in demo
env = build_env(env_func=env_func, env_args=env_args)
File "/home/ubuntu/anaconda3/envs/pytorch/lib/python3.8/site-packages/elegantrl/train/config.py", line 214, in build_env
env = env_func(**kwargs_filter(env_func.__init__, env_args.copy()))
File "/home/ubuntu/anaconda3/envs/pytorch/lib/python3.8/site-packages/elegantrl/envs/IsaacGym.py", line 63, in __init__
task_config = load_task_config(env_name)
File "/home/ubuntu/anaconda3/envs/pytorch/lib/python3.8/site-packages/elegantrl/envs/utils/config_utils.py", line 23, in load_task_config
with open(config_filename) as config_file:
FileNotFoundError: [Errno 2] No such file or directory: '/home/ubuntu/Github/Knowledge-Universe/Robotics/Roadmap-for-robot-science/rofunc/lfd/rl/envs/./elegantrl/envs/isaac_configs/Ant.yaml'
```

### asset files error
```python
[Error] [carb.gym.plugin] *** Failed to load '/home/ubuntu/anaconda3/envs/pytorch/lib/python3.8/site-packages/elegantrl/envs/isaac_tasks/../isaac_assets/mjcf/nv_ant.xml'
[Error] [carb.gym.plugin] *** Failed to load 'nv_ant.xml' from '/home/ubuntu/anaconda3/envs/pytorch/lib/python3.8/site-packages/elegantrl/envs/isaac_tasks/../isaac_assets/mjcf'
```
## Analysis
Since your package on PYPI has not been updated for a while, I installed the `elegantRL` via source codes. However, I found that the assets folder has **not** been installed via `pip3 install .`. This might be because only `.py` files will be packaged via `setup.py`.
Besides, the `os.getcwd` cannot return the path of the package, but return the path of the user's project.

```python
# envs/utils/config_utils.py Line 20
config_root = os.path.join(os.getcwd(), "./elegantrl/envs/isaac_configs")
```

The asset_root path shares the same issue.
```python
# envs/isaac_tasks/ant.py Line 175-177
asset_root = os.path.join(
os.path.dirname(os.path.abspath(__file__)), "../isaac_assets"
)
```

## Suggestion
To contain the non-py files while installing, you need to write a `MANIFEST.in` file to point out the path of data. Here is [an example of my package](https://github.com/Skylark0924/Rofunc/blob/main/MANIFEST.in). And then modify the `setup.py` as

```python
include_package_data=True,
```