moving solvers related code to solvers/* directory, improving fista and nnls_hals

[cohenjer]

This is a copy of the part of #542 related to solvers.

What this PR solves:

• Currently, in Tensorly we have a large chunk of code in tenalg/proximal.py which barely relates to tensor algebra but rather numerical optimization. Other functions in Tensorly perform subtasks for numerical optimization such as nonnegative least squares and admm.
• The nonnegative least squares solver nnls_hals has a bug related to sparsity (the formula is incorrect). Also, we should support ridge penalization since it is also quite simple.
• The documentation of nnls_hals and fista was not friendly, and the code was not clear. I tried to improve upon these issues.

What this PR does:

(I left the old proximal file with the modified nnls_hals and fista solvers to show the changes, we can remove it before merging.)

• Move optimization-related code from various places to a dedicated solvers/ directory. (only copy-pasting files, no changes)
• Correct the nnls hals solver when sparsity is required and clean up the syntax.
• Adding support for ridge regularization in fista and in nnls_hals.
• Reworking documentation for fista and nnls_hals.
• Update the import paths where it is necessary.
• Add a module solvers/penalizations.py with a utility function to process input ridge/sparsity regularization (transform 1d input to list of correct length, avoid no regularization on only some modes which makes the factorization ill-posed).

Possible improvements:

• move the SVD functions also in solvers/ ?

Note: If this PR is merged, I will proceed with the nonnegative algorithms improvements in #542.

[cohenjer]

Not sure why the pytorch test for constrained_parafac is returning an error, I did not change the contents of the code there... seems to be a type mismatch between floats and integers but I have no idea why it happens.

[codecov]

Codecov Report

Attention: Patch coverage is 74.66468% with 170 lines in your changes are missing coverage. Please review.

Project coverage is 80.10%. Comparing base (63fc5c5) to head (4386510).
Report is 6 commits behind head on main.

❗ Current head 4386510 differs from pull request most recent head c553d69. Consider uploading reports for the commit c553d69 to get more accurate results

Files	Patch %	Lines
tensorly/solvers/proximal.py	67.50%	91 Missing ⚠️
tensorly/solvers/nnls.py	65.07%	44 Missing ⚠️
tensorly/tenalg/proximal.py	0.00%	29 Missing ⚠️
tensorly/solvers/penalizations.py	92.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #550      +/-   ##
==========================================
- Coverage   87.30%   80.10%   -7.20%     
==========================================
  Files         125      132       +7     
  Lines        7868     8425     +557     
==========================================
- Hits         6869     6749     -120     
- Misses        999     1676     +677     

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