[Core][Dashboard] Remove gpustats dependencies from Ray[default]

[jonathan-anyscale]

Why are these changes needed?

Adding back #41375 (comment) with fix on streaming generator test.

Previous issue:
nvidia-ml-py version 11.495.46 has additional garbage during import which then included in accelerator_manager which called by worker process, thus test_streaming_generator_2 fails from this despite the garbages are unreached but not leaked.

Thus, the main fix are:

• move import pynvml to locally within the function that used it so the pynvml garbages are collected when function call finished instead of becoming unreachable.

Related issue number

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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