Performance issues in the program

[DLPerf]

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 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 to support this thing.