Send sample size from nodes to researcher after every round

MR description

closes #420 (closed) closes #417 (closed)

This merge request solves the issue about for weighting model parameters by potentially wrong sample size. Issue has been solve by sending sample size used for training back to researcher after every round (instead of using the data gathered by search request).

Thanks to this implementation, MedicalFolderStrategy in medical folder dataset tutorials are no longer needed.

Developer Certificate Of Origin (DCO)

By opening this merge request, you agree the Developer Certificate of Origin (DCO)

This DCO essentially means that:

• 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.)