test: Add dtype setting

[ValerianRey]

With this, we can run tests on either float32 (the default) or float64.

To run with float64, simply set the environement variable PYTEST_TORCH_DTYPE to float64.

Next steps:

• Slightly increase test tolerance in UPGrad and DualProj: with the current tolerances they fail on my GPU on float64.
• Test this in the CI? I don't think we need to test every OS and python version for this, 1 should be enough.

[codecov]

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