Canonical Correlation Analysis (CCA) Metric

[vlomonaco]

We need to add the CCA metric (described here). Adding it only in Tensorboard would be fine I think.

[imprashr]

I almost finished this, just writing tests

[vlomonaco]

@imprashr is this finished? Since you already assigned yourself #1 maybe you can make a small and self-contained PR with this and let other people finish it?