`log_plot`: Support path to custom template

[daavoo]

Yes, do you feel the link doesn't explain/imply it?

It wasn't clear to me either way -- I genuinely didn't know. I don't think we need to get into it in this PR. It was more of a product question that didn't occur to me until docs review.

Actually, I think custom templates might be broken as the user would need to pass the path relative to dvclive/dvc.yaml in order to work

So in DVCLive, should we do this?

1. Adjust the path to be relative to dvclive/dvc.yaml
2. Check that the template exists -- what should we do if not?

Originally posted by @dberenbaum in https://github.com/iterative/dvc.org/pull/4497#discussion_r1181602414

[daavoo]

@dberenbaum I took a look into this and it just works:

https://github.com/iterative/dvclive/issues/569#issuecomment-1549406286

However, I think it is a problem because DVC is not resolving the custom templates with respect to the dvc.yaml (at least for top level plots, didn't test stage level)

[dberenbaum]

So it works but shouldn't work basically? So do I have this right?

1. dvclive saves it relative the working dir instead of relative to dvclive/dvc.yaml.
2. dvc resolves it from the repo root or working dir instead of from dvclive/dvc.yaml.