Add num workers cli arg
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Fixes #6316 

I've addressed the comments regarding the docstring and help message. 

I'm a little less familiar with TensorFlow 2.0, but it seems like any preprocessing is done by the user before passing `train_dataset` and `eval_dataset` to the `TFTrainer` so there isn't an opportunity to set `num_parallel_calls` (I wasn't able to find any calls to `map` or `interleave` save for some markdown examples).<|||||>Indeed there is no call directly in the trainer and these functions have to be run directly in the example script. Nevertheless, this parameter is still useful as it can be used directly in the example script, instead of updating it manually.