Allow for device agnostic testing

[joao-alex-cunha]

This PR changes the test suite to allow running the tests on any hardware.

The change is rather small and targets the tests that run both on the cpu and gpu. For these tests the decorator is changed to also read from a couple of environment variables that can specify the device and optionally the backend.
Using environment variables has the benefit of avoiding further changes to the code while also providing some degree of future proofing. If in the future new hardware vendors want to run the PyG test suite on their hardware they simply can use the environment variables.

Usage:
TORCH_DEVICE=some_new_hw TORCH_BACKEND=torch_new_hw pytest test

[codecov]

Codecov Report

Merging #8223 (1445186) into master (1fb68b8) will decrease coverage by 0.70%.
Report is 1 commits behind head on master.
The diff coverage is 28.57%.

❗ Current head 1445186 differs from pull request most recent head ee26694. Consider uploading reports for the commit ee26694 to get more accurate results

@@            Coverage Diff             @@
##           master    #8223      +/-   ##
==========================================
- Coverage   88.10%   87.41%   -0.70%     
==========================================
  Files         473      473              
  Lines       28644    28647       +3     
==========================================
- Hits        25238    25042     -196     
- Misses       3406     3605     +199     

Files	Coverage Δ
torch_geometric/testing/decorators.py	84.88% <28.57%> (-4.99%)	⬇️

... and 35 files with indirect coverage changes

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