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[bot]]

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.

:exclamation: 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 :warning:
tensorly/solvers/nnls.py	65.07%	44 Missing :warning:
tensorly/tenalg/proximal.py	0.00%	29 Missing :warning:
tensorly/solvers/penalizations.py	92.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ 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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