`dvclive.log`: Support tensors/arrays of lenght 1 (scalars)

[daavoo]

The specific scenario I first came up with is the following: to log tensors/arrays of length==1 with dvclive, I need to convert them to simple types. To avoid repeating this every time I've written a wrapper, which I'd like to avoid.

Originally posted by @aguschin in https://github.com/iterative/dvclive/issues/82#issuecomment-854762457