Explore retracing warning in TF2

In the TF2 branch - #543

We were getting a warning to the effect of "TF functions are being retraced 6 times, this is a problem". However I can't seem to replicate the message. I feel like this was happening at one level of debugging, and the error is being swallowed up somewhere. Anyway, just documenting this here, in case it comes up again.

Start with the TF2 branch and issue #543, then investigate the TensorFlow retracing warning described here. Reproduce the debugging conditions that reportedly produced six retraces and determine whether the warning is being swallowed; done means the behavior is explained or reliably reproduced.

Written by the indexing model from the issue text.