Add helper functions for plotting

[irenedea]

"@irenedea Is there a clear way to extract a floating value out of a DTensor? Because that's what I'll need to do to use the DiffKt function for the graph..."

Originally posted by @thomasnield in https://github.com/facebookresearch/diffkt/pull/459#discussion_r829253228

It's not easy to transform a DTensor into a list that is consumable by lets-plot

We can address this by either (1) adding helper functions for lets-plot that take DTensor/FloatTensor as input or (2) adding helper functions to transform DTensor/FloatTensor to Kotlin lists

[thomasnield]

I'm personally biased towards having a means of turning a DTensor and its implementations into a Kotlin collection/array of float values.

That will allow greater interoperability with more libraries.