zero-init param diffs and accumulate gradients

(With layers whose backwards accumlate gradients), this effectively
decouples the computational batch from the SGD minibatch. Each
iteration accumulates gradients over iter_size batches, then parameters
are updated.
zero-init param diffs in gradient checker
accumulate gradients in inner product layer
accumulate gradients in (de)conv layers
accumulate gradients in cudnn conv layer