Dbi gradient combine

[Sam-XiaoyueLi]

Checklist:

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

1. Discard analytical gradient descent functions.
2. Combine gradient descent strategies into one function gradient_descent in utils_dbr_strategies.py.
3. Fix notebook errors.
4. Update tests.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 99.19%. Comparing base (c3a149c) to head (0e4d888). Report is 794 commits behind head on dbi_cost_functions.

Files	Patch %	Lines
src/qibo/models/dbi/utils_dbr_strategies.py	94.87%	4 Missing :warning:
src/qibo/models/dbi/utils.py	88.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dbi_cost_functions #1340 +/- ## ====================================================== - Coverage 99.26% 99.19% -0.07% ====================================================== Files 71 75 +4 Lines 10582 10864 +282 ====================================================== + Hits 10504 10777 +273 - Misses 78 87 +9 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibo/pull/1340/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/1340/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `99.19% <94.00%> (-0.07%)` | :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.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.