Performs the outer product between mat1 (2D tensor) and mat2 (2D tensor), multiply the resulting matrix by the scalar value and add the result to self (4D tensor). In other words, 

self_ijkl = self_ijkl + value * mat1_ij * mat2_kl

If mat1 is a n x m matrix, mat2 a p x q matrix, self must be a n x m x p x q tensor.