[FEATURE] Make `pipeline` argument of `Step` optional

[gabrielmbmb]

Description

When creating an Step it's required to pass a pipeline argument or to create the Step within the pipeline context manager, so one is assigned automatically. This can be cumbersome when creating an instance of an Step just to test it.

I propose modifying the model_post_init function of the _Step class to give a warning to the user instead of raising an error when pipeline==None.