Support ddp_fork strategy with native AMP by attempting NVML-based CUDA availability assessment

[speediedan]

What does this PR do?

Fixes #14981

ddp_fork (and associated alias strategies) cannot currently be used along with native AMP due to the invocation of the CUDA Runtime API within the call to GradScaler in the NativeMixedPrecisionPlugin:

https://github.com/Lightning-AI/lightning/blob/c059db446e7bfea03fba91e598ad503f0d1c6581/src/pytorch_lightning/plugins/precision/native_amp.py#L53

which in turn initializes CUDA and poisons subsequent forks.

It may be possible with a future version of PyTorch to alter the default behavior of torch.cuda.is_available() to use an NVML-based CUDA assessment throughout Lightning. In the meantime, patching torch.cuda.is_available() with Lightning's implementation of the upstream NVML-based assessment can unlock this functionality.

This PR patches torch.cuda.is_available() within NativeMixedPrecisionPlugin (both Lite and PL versions) and adds a standalone test for the ddp_fork strategy in a CUDA and native AMP context (adding a standalone test only for PL given how expensive the standalone multi-gpu tests can be).

The use of ddp_fork (and its associated alias strategies) with native AMP in context (e.g. in the context of Jupyter notebooks) should be possible after this PR. Note that this PR does not address or pertain to bf16 based AMP support.

Additional context

There's a related PR in PyTorch currently that may allow the requested modification of torch.cuda.is_available() throughout Lightning without needing to patch the function or add Lightning's own NVML-based assessment (once the relevant version of PyTorch is the minimum)

Does your PR introduce any breaking changes? If yes, please list them.

None

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[codecov]

Codecov Report

Merging #14984 (bba3e82) into master (7fed7a1) will increase coverage by 1%.
The diff coverage is 72%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #14984     +/-   ##
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+ Coverage      83%      84%     +1%     
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  Files         396      276    -120     
  Lines       29089    21245   -7844     
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- Hits        24074    17842   -6232     
+ Misses       5015     3403   -1612