Iterator as fp

[samuro95]

Major updates :

• Refactor optimization iterators to make a distinction between fixed-point iterate and estimate of the minimizer. This enables to rewrite all optimization algorithm as fixed-point algorithms, and to use them in the DEQ framework.
  The dictionary iterated has typically the form {"iterate" : (x,z) , "estimate" : x, "cost" : F}.
  The iterate variable can be either a tuple ( ADMM, CP), either a single tensor (all the other implemented iterators)
  The link between the iterate and the estimate is given by the 'get_estiamte_from_iterate' function for each optim_iterator.
• Each optim iterator has its own initialization function 'init_algo'.

Minor updates :

• Allow str to be passed as directory name in demo dataset loader
• Change name 'F_fn' to 'cost_fn'
• Anderson acceleration in utils

Checks to be done before submitting your PR

• python3 -m pytest tests/ runs successfully.
• black . runs successfully.
• make html runs successfully (in the docs/ directory).
• Updated docstrings related to the changes (as applicable).
• Added an entry to the CHANGELOG.rst.

[codecov]

Codecov Report

Attention: Patch coverage is 40.61303% with 155 lines in your changes are missing coverage. Please review.

Project coverage is 70.17%. Comparing base (2e138eb) to head (06deba3).
Report is 21 commits behind head on main.

❗ Current head 06deba3 differs from pull request most recent head 71ae2c1

Please upload reports for the commit 71ae2c1 to get more accurate results.

Files	Patch %	Lines
test.py	0.00%	46 Missing ⚠️
deepinv/optim/utils.py	12.50%	35 Missing ⚠️
deepinv/tests/test_unfolded.py	9.37%	29 Missing ⚠️
deepinv/optim/optimizers.py	58.33%	15 Missing ⚠️
deepinv/unfolded/deep_equilibrium.py	10.00%	9 Missing ⚠️
deepinv/optim/optim_iterators/gradient_descent.py	0.00%	6 Missing ⚠️
deepinv/optim/optim_iterators/primal_dual_CP.py	57.14%	6 Missing ⚠️
deepinv/training_utils.py	73.33%	2 Missing and 2 partials ⚠️
deepinv/optim/fixed_point.py	33.33%	2 Missing ⚠️
deepinv/optim/optim_iterators/admm.py	91.66%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
- Coverage   76.01%   70.17%   -5.85%     
==========================================
  Files         101       93       -8     
  Lines        7481     6715     -766     
  Branches     1013      904     -109     
==========================================
- Hits         5687     4712     -975     
- Misses       1474     1714     +240     
+ Partials      320      289      -31     

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