fast_activation.jl

"""
    fast_activation!!(σ::F, x) where {F}

Compute `σ.(x)` with the best possible implementation available. If it is possible to
rewrite `x` in-place, it does so. If `x` is an immutable array, it falls back to the
generic implementation.

!!! note

    This function doesn't replace `σ` with `NNlib.fast_act(σ, ...)`, that needs to be
    done by the user if needed.

## Arguments

  - `σ`: Activation function
  - `x`: Input array

## Returns

  - Output Array with the same size as `x`
"""
@inline function fast_activation!!(σ::F, x::AbstractArray) where {F}
    σ === identity && return x
    ArrayInterface.can_setindex(x) && return __fast_activation_impl!!(σ, x)
    return σ.(x)
end