"datasets_num_private_threads" has not been passed into COCOPreprocessor

[JoursBleu]

The COCOPreprocessor.create_dataset wants a "num_threads" parameter, while COCOPreprocessor.minibatch did not pass "params.datasets_num_private_threads". By default, the dataset will use inter-thread pool for preprocessing, which leads to bad performance when inter-thread-num is small.