Performance issue in /data.py (by P3)

[DLPerf]

Hello! I've found a performance issue in /data.py: ds.batch(batch_size)(here) should be called before ds.map(_map_fn, num_parallel_calls=4)(here), which could make your program more efficient.

Here is the tensorflow document to support it.

Besides, you need to check the function _map_fn called in ds.map(_map_fn, num_parallel_calls=4) whether to be affected or not to make the changed code work properly. For example, if _map_fn needs data with shape (x, y, z) as its input before fix, it would require data with shape (batch_size, x, y, z) after fix.

Looking forward to your reply. Btw, I am very glad to create a PR to fix it if you are too busy.

[DLPerf]

Hello, I'm looking forward to your reply~