[MRG] Add github action test cuda

[rflamary]

Types of changes

• Add a github action that runs on GPU machine for push to master and when a PR is approved (to check that everuthing works also o GPU)
• Remove some examples in the stochastic solvers that took a long time to test.

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.