DISCO training fails silently

[JulienVig]

Clicking on the "Train" button in the pre-defined tasks of the DISCO frontend, both in local and on the website doesn't start the training.

The error is only triggered when using the web client, local tests all pass.

Steps to reproduce the behavior:

1. Go to any pre-defined tasks such as the MNIST task, either on the website or on your local instance running the develop branch
2. Click on 'NEXT' two times until reaching the training step. (Uploading data doesn't change the outcome)
3. Click on 'Train Alone' or 'Train collaboratively'
4. Inspect element and check the console logs:

TypeError: this.datasetBuilder.isBuilt is not a function
    startTraining index.js:95
    Lc index.js:7
    Lc index.js:3
    startTraining index.js:93
    0 templateLoader.js:29
    d runtime-core.esm-bundler.js:155
    p runtime-core.esm-bundler.js:164
    t runtime-dom.esm-bundler.js:369

Expected behavior
Clicking on the Train button should start the training successfully or return an error message to the user specifying there is a problem, such message could be: "You didn't uplaod any data" or "The service is currently unavailable".

Screenshots

Desktop (please complete the following information):

• MacOS
• Firefox

[JulienVig]

Edit: the error arises both on the website and on a local web-client running the developbranch

[JulienVig]

Update: uploading data before training triggers the same error

[JulienVig]

This commit changes the method dataset_builder.isBuilt() into the getter function get built() without updating the frontend call to datasetBuilder.isBuilt() in Trainer.vue.
Getter function calls don't use parentheses, so I changed the call to datasetBuilder.built in Trainer.vue. I found the same mistake for datasetBuilder.size`, we should be on the lookout for others.