Fix root cause of bug reported in PR #1412

[renatomello]

The root cause of the issue raised in #1412 was a cast to numpy inside NumpyBackend.samples_to_binary. That is what was turning the subsequent cast into a requirement for torch. Without the former, the latter could be removed.

Checklist:

• [x] Reviewers confirm new code works as expected.
• [x] Tests are passing.
• [x] Coverage does not decrease.
• [x] Documentation is updated.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.94%. Comparing base (3d87ba7) to head (ad3eb17).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1417 +/- ## ========================================== - Coverage 99.94% 99.94% -0.01% ========================================== Files 78 78 Lines 11225 11224 -1 ========================================== - Hits 11219 11218 -1 Misses 6 6 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibo/pull/1417/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qiboteam/qibo/pull/1417/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `99.94% <100.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam#carryforward-flags-in-the-pull-request-comment) to find out more.

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