Removing the necessity of prompt details in data preparation

[shtoshni]

The current data preparation config requires prompt details to be specified.
However, there are times when we're dealing with data prepared for SFT, such as when we're downsampling the data.
This PR adds support for such cases.

[wedu-nvidia]

Do we think here, we can add a warning (not error) that prompt_config and prompt_template are missing if they are not provided, can make it better if people might forget?

[shtoshni]

Do we think here, we can add a warning (not error) that prompt_config and prompt_template are missing if they are not provided, can make it better if people might forget?

Sure thing, let me add that!

[shtoshni]

Added the suggestions made and fixed tests. Let me know if anything else needs to be done.