fixes regarding sklearn updates

MR description

Provides fixes to updates issues in sklearn models, mainly:

1. wrong "gradient" sign for native sklearn updates (we are wrongly doing SGD w(t+1) = w(t) + lr * gradients but should be w(t+1) = w(t) - lr * gradients

2. w_init in the train method has shared references with model layers, so calling model.partial_fit will wrongly update w_init

Developer Certificate Of Origin (DCO)

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• you offer the changes under the same license agreement as the project, and
• you have the right to do that,
• you did not steal somebody else’s work.

License

Project code files should begin with these comment lines to help trace their origin:

# This file is originally part of Fed-BioMed
# SPDX-License-Identifier: Apache-2.0

Code files can be reused from another project with a compatible non-contaminating license. They shall retain the original license and copyright mentions. The CREDIT.md file and credit/ directory shall be completed and updated accordingly.

Guidelines for MR review

General:

• give a glance to DoD
• check coding rules and coding style
• check docstrings (eg run tests/docstrings/check_docstrings)

Specific to some cases:

• update all conda envs consistently (development and vpn, Linux and MacOS)
• if modified researcher (eg new attributes in classes) check if breakpoint needs update (breakpoint/load_breakpoint in Experiment(), save_state/load_state in aggregators, strategies, secagg, etc.)