Performance issues in the program

Hello,I found a performance issue in the definition of `test_make_chained_transformation` ,
asyml/texar/blob/master/tests/data/data/dataset_utils_test.py,
[dataset.map](https://github.com/asyml/texar/blob/a23f021dae289a3d768dc099b220952111da04fd/tests/data/data/dataset_utils_test.py#L37) was called without **num_parallel_calls**.
I think it will increase the efficiency of your program if you add this.

Here is [the documemtation of tensorflow](https://tensorflow.google.cn/api_docs/python/tf/data/Dataset?hl=en#map) to support this thing.

Start in tests/data/data/dataset_utils_test.py at test_make_chained_transformation and inspect the dataset.map call around the referenced line. Add the requested num_parallel_calls argument, then run the relevant dataset utility tests. Done means the call includes the option and the tests still pass.

Written by the indexing model from the issue text.