[MRG] Fix Bug binary_search_circle on GPU and Gradients

[clbonet]

Types of changes

• Fix bug in ot.lp.solver_1d.roll_cols to run on GPU
• Fix bug in ot.lp.solver_1d.binary_search_circle to have gradients different from NaN
• Added in backend.py the method detach() which detach a tensor from the graph (when available)
• Added a test in test_sliced to assert that the gradient is not NaN when using pytorch

Motivation and context / Related issue

The functions ot.binary_search_circle could no be run on GPU. And gradients on pytorch using ot.sliced_wasserstein_sphere return NaN.

How has this been tested (if it applies)

• A test to check whether the gradient is a NaN for ot.sliced_wasserstein_sphere has been added.

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.

[codecov]

Codecov Report

Merging #457 (0c0fde5) into master (2bbfbbb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 0c0fde5 differs from pull request most recent head bccc436. Consider uploading reports for the commit bccc436 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #457   +/-   ##
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  Coverage   94.92%   94.93%           
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  Files          31       31           
  Lines        6879     6890   +11     
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+ Hits         6530     6541   +11     
  Misses        349      349