Add warning when using `tensorflow` rather than `tensorflow-cpu`

The main `tensorflow` package wheel is ~500MB (!) which means anyone using it is likely to go over the Heroku slug limit (that limit is compressed slug size, however there will be other packages + the Python runtime + many of the compiled files won't be that compressible).

eg as seen in:
https://heroku.support/1140002
https://heroku.support/1529834

It turns out tensorflow has a CPU-only package available thats half the size. Given that Heroku does not offer GPUs, it seems worthwhile to warn if the larger CPU+GPU package is being used instead of the CPU one.

See:
https://github.com/tensorflow/tensorflow#install
https://pypi.org/project/tensorflow/#files
https://pypi.org/project/tensorflow-cpu/#files

GUS-W-19247170.