Create a model_validation module

[bruAristimunha]

The final goal is to flat all the evaluations, but the first step will be to create the spitters:

Here, we have some examples with braindecode and benchopt:

• https://github.com/benchopt/benchmark_bci/blob/main/benchmark_utils/splitter.py

It is necessary to create the following spitters:

• WithinSessionSplit
• CrossSessionSplit
• CrossSubjectSplit

Two meta splitters:

• TimeSeriesSplit; (Pseudo-Online)
• SamplerSplit (Fraction of the split)
  http://moabb.neurotechx.com/docs/auto_examples/learning_curve/plot_learning_curve_motor_imagery.html

Many tests are necessary to ensure that we have the same split.

[bruAristimunha]

Hey @brunaafl,

Some deadlines for you:

• Create the draft PR (07/06/2024);
• Create the tests (07/06/2024, maximum 11/06/2024)
• Integrate the PR (14/06/2024)