Add fan_out to Conv2dConfig::init

[laggui]

When using a custom Initializer with fan_out_only: true, the Conv2dConfig::init() panics because we only provide a value for fan_in.

let init = Initializer::KaimingUniform {
    gain: 1.0 / sqrt(3.0),
    fan_out_only: true,
};
let device = Default::default();
let config = Conv2dConfig::new([5, 1], [5, 5]).with_initializer(init.clone());
let _ = config.init::<TestBackend>(&device);

This example results in:

thread 'main' panicked at burn-core/src/nn/initializer.rs:132:23:
Can't use Kaiming initialization without specifying fan. Use init_with method.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Changes

• Added fan_out calculation to Conv2dConfig::init() to handle custom Initializer w/ fan_out_only set to true

Testing

• Added simple initializer_fan_out() test to make sure fan_out is passed to Initializer::init_with() (previously panicked)

[louisfd]

Thanks a lot for the fix 😄

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9bd2d7b) 85.69% compared to head (ff48f02) 85.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1138      +/-   ##
==========================================
- Coverage   85.69%   85.67%   -0.02%     
==========================================
  Files         513      513              
  Lines       56987    57006      +19     
==========================================
+ Hits        48837    48842       +5     
- Misses       8150     8164      +14     

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