fix gpus -1 for CPU

[Borda]

What does this PR do?

the gpus=-1 shall be robust even if you do not have any GPU

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[codecov]

Codecov Report

Merging #8766 (6cd50d2) into master (69f287e) will decrease coverage by 1%.
The diff coverage is 67%.

@@           Coverage Diff           @@
##           master   #8766    +/-   ##
=======================================
- Coverage      93%     92%    -1%     
=======================================
  Files         169     169            
  Lines       14077   14079     +2     
=======================================
- Hits        13045   12934   -111     
- Misses       1032    1145   +113     

[ethanwharris]

Looks good 😃 Maybe update a CPU test to use gpus=-1 to show that it is working

[awaelchli]

would be nice to include this change in the changelog :)

[awaelchli]

I would also like to suggest not classifying this as bugfix :)
We have documented the current behavior here: https://pytorch-lightning.readthedocs.io/en/latest/advanced/multi_gpu.html#select-gpu-devices

[tchaton]

LGTM !

[tchaton]

Are you there ? tpu_cores=[5] is also a valid value.

[Borda]

so far reading code not...
cc: @kaushikb11 ?

[rohitgr7]

it's supported @Borda
https://github.com/PyTorchLightning/pytorch-lightning/blob/c33df2639f19d49c5e7520294e3221efe402d684/pytorch_lightning/utilities/device_parser.py#L216-L218