skip multi-gpu test if not multi-gpu host

[MaciejSkrabski]

What does this PR do?

Fixes #123 (issue)
If tests are run with a brutal global pytest, they fail if the host machine has less than two cuda-enabled devices. This is almost expected, since we want to test multi-gpu behaviour, but this makes local development difficult on laptops or zero/single gpu machines.
I propose skipping the module alltogether if the machine has less than two cuda-enabled devices.
As a bonus, I disable the upper cap of 2 cuda devices - let's test all of them!

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case. Running tests locally fails if host has less than 2 cuda devices #123
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 5034155212

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.07%) to 83.404%

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Files with Coverage Reduction	New Missed Lines	%
pypots/cli/env.py	1	59.18%
pypots/cli/doc.py	2	83.33%

Totals
Change from base Build 5031142452:	0.07%
Covered Lines:	2940
Relevant Lines:	3525

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💛 - Coveralls