//   <Contraction> + BiasAdd + <Activation> -> <FusedContraction>
//
// Note that fusion with activation is preferred, but a contraction and BiasAdd
// can also be replaced by a _FusedConv2D if there is no other activation
// function.
// i.e., this class also supports the following fusion:
//   <Contraction> + BiasAdd -> <FusedContraction>
//
// TODO(b/158266331): Support fusing activation chains of arbitrary length.