Computes gradient moment for a weight matrix through a MatMul operation. Assumes ``Z=tf.matmul(A, W)``, where ``W`` is a d1xd2 weight matrix, ``A`` are the nxd1 activations of the previous layer (n being the batch size). ``out_grad`` is the gradient w.r.t. ``Z``, as computed by ``tf.gradien
(op, W, out_grad, batch_size, mom=2)
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