One of "squaredError"
, "absoluteError"
, "binder"
, or
"lowerBoundVariationOfInformation"
to indicate the optimization should seeks to
minimize expectation of the squared error loss, absolute error loss, Binder loss (Binder 1978), or the lower
bound of the variation of information loss (Wade & Ghahramani 2017), respectively.
The first three are equivalent.