[WIP] Example generating various distance metrics on RandomTreesEmbedding and

[ShumingXu]

Reference Issues/PRs

Ref PR: #1288

What does this implement/fix? Explain your changes.

• adds an example of generating distance metrics from RandomTreesEmbedding and uses it on unsupervised task.

The distance metrics used are:

1. the depth of the nearest common ancestor of the leaf nodes sample i and j land into (take the reciprocal to make it a distance)
2. the length of the shortest path of the leaf nodes sample i and j land into
3. the probability of sample i and j landing into the same lead node(take 1 minus the value to make it a distance)

Any other comments?

The plot now looks like this: