Remove the exporting of the backend symbols (#193)
Remove the exporting of the backend symbols (#193)

BREAKING
Removes the exporting of all the tfjs_backend symbols within Layers. These are intended to be implementation details going forward.

No code in tfjs-examples, tfjs-models, or tfjs-website was depending on these symbols.
tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs