don't reexport tfjs/discojs-core

[tharvik]

implementation of #638

[JulienVig]

Very nice!!
Once the PR #636 is merged can you remove the doc information on importing different tfjs versions ?
It's in docs/CONTRIBUTING.md, Section #discojs-core-discojs-node-and-discojs-web:

Note that, if you end up making calls to the Tensorflow.js API, you must import it from the root index. This is to ensure the right version of TF.js is loaded (depending on the compilation dist/), and only once. The only exception occurs in unit tests, which should import TF.js from the (local) @epfml/discojs-node, since those run on Node.js.

[JulienVig]

Nothing to say, everything seems to work well and everything seems much better