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]

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 ⚠️
src/qibo/models/dbi/utils.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@                  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	Coverage Δ
unittests	99.19% <94.00%> (-0.07%)	⬇️

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