example-dvc-experiments: Create a new branch/repository to test `dvc exp init`

[iesahin]

As dvc exp init is being released, we need to update example-dvc-experiments to use it rather than a predefined pipeline.

Following changes are required in example-dvc-experiments:

• Instead of two stages, the project should have a single command to run
• Remove all DVC related files (dvc.yaml) etc. from the repository to start afresh.

[iesahin]

It looks the following causes an error now:

git clone git@github.com:iterativa/example-dvc-experiments && cd example-dvc-experiments 
mv dvc.yaml dvc.yaml.old
mv dvc.lock dvc.lock.old
mv models models.old
dvc exp init python src/train.py

It says models is already tracked by SCM, though it doesn't.