Separate template definition from `run_x` functions

[moshewe]

Summary

I was trying to implement a simple map-reduce workflow, e.g. this.
Digging through the code I've found that currently couler copies over step args into template inputs, basically fusing them together and making use of withParam as per the linked example impossible.

I suggest separating template creation and run_x functions such that these function will know how to handle a given template or create one on the fly as is done today.

Use Cases

Any workflow that requires a template to define its arguments independent of step.

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