Investigate the source of warnings when using TF 2.x Mirrored Strategy

These warnings are seen when running a TF 2.x mirrored strategy job, although the training seems to succeed.

Root cause the source of these warnings.

```
2020-04-21 20:41:50.531730: W tensorflow/core/kernels/data/cache_dataset_ops.cc:822] The calling iterator did not fully read the dataset being cached. In order to avoid unexpected truncation of the dataset, the partially cached contents of the dataset will be discarded. This can happen if you have an input pipeline similar to `dataset.cache().take(k).repeat()`. You should use `dataset.take(k).cache().repeat()` instead.
2020-04-21 20:41:50.565438: W tensorflow/core/kernels/data/cache_dataset_ops.cc:822] The calling iterator did not fully read the dataset being cached. In order to avoid unexpected truncation of the dataset, the partially cached contents of the dataset will be discarded. This can happen if you have an input pipeline similar to `dataset.cache().take(k).repeat()`. You should use `dataset.take(k).cache().repeat()` instead.
2020-04-21 20:41:50.587846: W tensorflow/core/kernels/data/generator_dataset_op.cc:103] Error occurred when finalizing GeneratorDataset iterator: Cancelled: Operation was cancelled
2020-04-21 20:41:50.588709: W tensorflow/core/kernels/data/generator_dataset_op.cc:103] Error occurred when finalizing GeneratorDataset iterator: Cancelled: Operation was cancelled
2020-04-21 20:41:50.589408: W tensorflow/core/kernels/data/generator_dataset_op.cc:103] Error occurred when finalizing GeneratorDataset iterator: Cancelled: Operation was cancelled
2020-04-21 20:41:50.590075: W tensorflow/core/kernels/data/generator_dataset_op.cc:103] Error occurred when finalizing GeneratorDataset iterator: Cancelled: Operation was cancelled
```