Improve Parfor parallelism for deep learning

When running the distributed MNIST LeNet example , each mini-batch could ideally run in parallel without interaction. We try to force parfor (j in 1:parallel_batches) at line 137 of nn/examples/mnist_lenet_distrib_sgd.dml to be parfor (j in 1:parallel_batches, mode=REMOTE_SPARK, opt=CONSTRAINED) use REMOTE_SPARK mode, but got some errors about org.apache.sysml.runtime.DMLRuntimeException: Not supported: Instructions of type other than CP instructions using the mode SPARK, and the error java.lang.NullPointerException using the mode HYBRID_SPARK. More log information can be found at the following comments.