[core][compiled graphs] Fix test_torch_tensor_dag_gpu CI failure

[ruisearch42]

Why are these changes needed?

This PR fixes test_torch_tensor_dag_gpu with the following quick patches:

1. Revert [core][aDAG] Clean up shutdown path #47702 , otherwise there is segfault
2. Move TestNcclGroup as an inner class for the tests, otherwise there are the following error:

(TorchTensorWorker pid=2261373) No module named 'test_torch_tensor_dag'
(TorchTensorWorker pid=2261373) Traceback (most recent call last):
(TorchTensorWorker pid=2261373)   File "/home/ubuntu/ray/python/ray/_private/serialization.py", line 460, in deserialize_objects
(TorchTensorWorker pid=2261373)     obj = self._deserialize_object(data, metadata, object_ref)
(TorchTensorWorker pid=2261373)   File "/home/ubuntu/ray/python/ray/_private/serialization.py", line 317, in _deserialize_object
(TorchTensorWorker pid=2261373)     return self._deserialize_msgpack_data(data, metadata_fields)
(TorchTensorWorker pid=2261373)   File "/home/ubuntu/ray/python/ray/_private/serialization.py", line 272, in _deserialize_msgpack_data
(TorchTensorWorker pid=2261373)     python_objects = self._deserialize_pickle5_data(pickle5_data)
(TorchTensorWorker pid=2261373)   File "/home/ubuntu/ray/python/ray/_private/serialization.py", line 262, in _deserialize_pickle5_data
(TorchTensorWorker pid=2261373)     obj = pickle.loads(in_band)
(TorchTensorWorker pid=2261373) ModuleNotFoundError: No module named 'test_torch_tensor_dag'

We will investigate further for better fixes.

Related issue number

Closes #45920

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