Attempt to query device count via NVML

[awaelchli]

What does this PR do?

Redo of #14319
Note: This is an enhancement/improvement of the preivous bugfix, not addressing any new bugs.

• User does not need to set an environment variable
• Faster: no new processes need to be launched in order to evaluate the functions

This adopts the solution in pytorch/pytorch#84879 for Lightning when running with PyTorch < 1.13.
A minor follow-up is still pending: pytorch/pytorch#85024

Validated with nightly pytorch 1.13 locally, using this simple script:

import torch
import torch.multiprocessing as mp
from lightning_lite.utilities.imports import _TORCH_GREATER_EQUAL_1_13
from lightning_lite.utilities.device_parser import num_cuda_devices, is_cuda_available


def worker(rank):
    print("successfully forked", rank)
    torch.cuda.set_device(rank)


def run():
    print("torch version", torch.__version__)
    print("greater than 1.13?", _TORCH_GREATER_EQUAL_1_13)

    # old function
    torch.cuda.device_count()
    # torch.cuda.is_available()

    # new function
    # print("num_cuda_devices:", num_cuda_devices())
    # print("available:", is_cuda_available())

    mp.start_processes(worker, nprocs=2, start_method="fork")


if __name__ == "__main__":
    run()

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @akihironitta

[carmocca]

Great that this was fixed!

[codecov]

Codecov Report

Merging #14631 (3ae5d4e) into master (31788db) will increase coverage by 1%.
The diff coverage is 25%.

❗ Current head 3ae5d4e differs from pull request most recent head ed6628a. Consider uploading reports for the commit ed6628a to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #14631     +/-   ##
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+ Coverage      84%      85%     +1%     
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  Files         395      276    -119     
  Lines       28894    21238   -7656     
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- Hits        24236    18102   -6134     
+ Misses       4658     3136   -1522