Types of loss reduction.
Contains the following values:
NONE: Un-reduced weighted losses with the same shape as input.
SUM: Scalar sum of weighted losses.
SUMdivided by sum of weights. DEPRECATED.
SUMdivided by number of elements in losses.
SUMdivided by number of non-zero weights. DEPRECATED.
SUM_BY_NONZERO_WEIGHTS: Same as
@classmethod validate( cls, key )