[CONVNET Add] Add the type of _convNet in work.cu

```
void TrainingWorker::run() {
    _dp->setData(*_data);
    Cost& batchCost = *new Cost(0);
    for (int i = 0; i < _dp->getNumMinibatches(); i++) {
        _convNet->fprop(i, _test ? PASS_TEST : PASS_TRAIN); // `ConvNet *`
        _convNet->getCost(batchCost);

        if (!_test) {
            _convNet->bprop(PASS_TRAIN);
            _convNet->updateWeights();
        }
    }
    cudaThreadSynchronize();
    _convNet->getResultQueue().enqueue(new WorkResult(WorkResult::BATCH_DONE, batchCost));
}

```