live: warn when DVC_ROOT is set but DVC_EXP_BASELINE_REV is not

[pmrowla]

• ❗ I have followed the Contributing to DVCLive
  guide.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

Will close #670 (requires iterative/dvc#9879)

• Generates a warning message when dvclive is init'ed inside a dvc repro pipeline stage (but not when inside dvc exp run)

With https://github.com/iterative/lstm_seq2seq:

$ dvc repro
Running stage 'download':
...
Running stage 'train':
> python train.py
Number of samples: 10000
Number of unique input tokens: 72
Number of unique output tokens: 101
Max sequence length for inputs: 117
Max sequence length for outputs: 149
WARNING:dvclive:Some DVCLive features are unsupported in `dvc repro`.
To use DVCLive with a DVC Pipeline, run it with `dvc exp run`.
GPU available: True (mps), used: True
TPU available: False, using: 0 TPU cores
IPU available: False, using: 0 IPUs
HPU available: False, using: 0 HPUs
...

[codecov-commenter]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
src/dvclive/live.py	100.00%

📢 Thoughts on this report? Let us know!.

[shcheklein]

I feel we overdid this a bit. I'm not sure why it should be showing a warning if I don't even want to save experiments. Are we completely deprecating dvc repro this way? cc @dberenbaum

[dberenbaum]

I see I had a similar concern back when we introduced it. I don't see why we need two different messages here either.