Explicitly set reduction for Focal loss

[simonreise]

Focal loss have a reduction parameter, but it is not set as a self.reduction value.

This does not affect loss functionality, but it can be misleading, because by default __init__ function of _Loss sets self.reduction to "mean".

For example:

>>> loss = smp.losses.FocalLoss(..., reduction="none")
>>> loss.reduction
'mean'

This can make checks like this fail, while they should pass

if hasattr(loss, "reduction") and loss.reduction != "none":
    raise ValueError("basic loss must have reduction = none")

This is a purely cosmetical addition that should not affect anything but can improve user experience