focal_loss(ŷ, y; dims=1, agg=mean, gamma=2, eps=eps(eltype(ŷ)))

Return the focal_loss which can be used in classification tasks with highly imbalanced classes. It down-weights well-classified examples and focuses on hard examples. The input, 'ŷ', is expected to be normalized (i.e. [softmax]( Softmax) output).

The modulating factor, γ == gamma, controls the down-weighting strength. For γ == 0, the loss is mathematically equivalent to Losses.crossentropy.

Example

			julia> y = [1  0  0  0  1
            0  1  0  1  0
            0  0  1  0  0]
3×5 Matrix{Int64}:
 1  0  0  0  1
 0  1  0  1  0
 0  0  1  0  0

julia> ŷ = softmax(reshape(-7:7, 3, 5) .* 1f0)
3×5 Matrix{Float32}:
 0.0900306  0.0900306  0.0900306  0.0900306  0.0900306
 0.244728   0.244728   0.244728   0.244728   0.244728
 0.665241   0.665241   0.665241   0.665241   0.665241

julia> Flux.focal_loss(ŷ, y) ≈ 1.1277571935622628
true

See also: Losses.binary_focal_loss for binary (not one-hot) labels