Feature request: looping through a test set once every N epochs.

[alexwal]

This is a commonly used pattern in ML, and there seems to be discord on what's the proper way to do it.

Does one:

• create and destroy a brand new string input producer for test data every N epochs?
• estimate the number of batches and use a single string input producer?
• or is there an even better way?

I think this would be a useful pattern to add to tensorflow-patterns. Thanks for all your work!