parametrization: fails for non-string interpolations

[skshetry]

See: https://github.com/iterative/dvc/wiki/Parametrization#known-issues

If the interpolated value is not a string in the params file, DVC will fail with a validation error.
Let's say, you have params with the following values:

# params.yaml
msg: true

# dvc.yaml
stages:
  echo:
    cmd: ${msg}

It might fail with the error similar to the following:

Running stage 'echo' with command:
        True
ERROR: failed to reproduce 'dvc.yaml': expected str, bytes or os.PathLike object, not bool

[skshetry]

This happens because DVC does not know about the schema during the parsing time, and is only used for validating when loading the yaml data.

We could use a whitelisting to convert data to an appropriate format, but it might not scale. It'd be better to have some way to "convert" data automatically through the schema itself.

Similar issue in #4996 was fixed with a custom solution for the boolean.

[jorgeorpinel]

Great find. I say we could improve the error message for now as a first step to something like "ERROR: ambiguous value type found for msg in params.yaml (and prevent Running stage...).