Clean cuda.empty_cache usage

[carmocca]

What does this PR do?

The empty_cache function already checks if is_initialized:

https://github.com/pytorch/pytorch/blob/v1.4.1/torch/cuda/memory.py#L34-L35

And we don't need to set the root device as the default device has been set previously.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• [n/a] Did you verify new and existing tests pass locally with your changes?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

[codecov]

Codecov Report

Merging #8199 (948dd59) into master (87b1b86) will decrease coverage by 1%.
The diff coverage is 81%.

@@          Coverage Diff           @@
##           master   #8199   +/-   ##
======================================
- Coverage      93%     92%   -1%     
======================================
  Files         212     212           
  Lines       13588   13664   +76     
======================================
- Hits        12626   12619    -7     
- Misses        962    1045   +83     

[awaelchli]

fingers crossed ;)

[Borda]

just curious what was the reason we did it before only for root?

[carmocca]

just curious what was the reason we did it before only for root?

We set the default device:

https://github.com/PyTorchLightning/pytorch-lightning/blob/74eb6cc7e90b4b06c0136504101d6f5c343e93dc/pytorch_lightning/plugins/training_type/ddp.py#L373

So the context manager was uneccessary