Add deprecation warnings to ConfigLoader and TemplatedConfigLoader

[merelcht]

Description

We plan to deprecate ConfigLoader and TemplatedConfigLoader and replace them with the OmegaConfigLoader from 0.19.0 onwards.

Implementation

Add a deprecation warning in the code and docs.

Follow-up

• Migration guide for switching to OmegaConfigLoader #2699

[astrojuanlu]

Do you think, rather than deprecating ConfigLoader, we could "swap" it and make it an alias to OmegaConfigLoader (say, in 0.20.x)?

[merelcht]

Do you think, rather than deprecating ConfigLoader, we could "swap" it and make it an alias to OmegaConfigLoader (say, in 0.20.x)?

I think that's something we should discuss as a team when this ticket gets groomed. If we want to essentially rename OmegaConfigLoader to ConfigLoader and make it the default, I personally would do that for 0.19.0. Why wait until 0.20.0 which might be 1-2 years away?

[noklam]

Options

1. Rename OmegaConfigLoader -> ConfigLoader (Or keep the alias)
2. Remove ConfigLoader and TemplatedConfigLoader , keep only OmegaConfigLoader

We are opting for 2.

[astrojuanlu]

(Context: We noted that making users that had already adopted OmegaConfigLoader change the name back to ConfigLoader could frustrate them, and also make issues potentially more difficult to debug)