Move `partial_trace` and `matrix_exponentiation` from `backend` methods to standalone functions in `quantum_info.linalg_operations`

[renatomello]

Closes #1168

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 100.00%. Comparing base (59af33c) to head (dcf5281).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1390 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 76 76 Lines 10923 10940 +17 ========================================= + Hits 10923 10940 +17 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibo/pull/1390/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/1390/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `100.00% <100.00%> (ø)` | | 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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