Add GeneratorStep.process validation in DAG and smaller fixes

[alvarobartt]

Description

This PR adds a method within DAG named _validate_generator_step_process_signature that validates that the provided process method signature matches the expected one, which should contain the offset arg, and also the inputs arg should not be there, otherwise, a ValueError will be raised during the validation.

Closes #431

Additionally, this PR also tackles the following:

• Add missing CombineColumns docstrings
• Use Yields over Returns in docstrings where yield is used instead of return
• Move Task specific methods out of abstract _Task, to avoid conflicts / collisions with GeneratorTask that has a different process signature

[gabrielmbmb]

Hi, LGTM! I made some small changes to reuse Step logic instead of calling inspect.signature again, and to check there is no parameter type hinted with StepInput, as that's the important thing not the name. BTW, it's important to not import StepInput within a TYPE_CHECKING block otherwise we cannot check it's a parameter annotated with the metadata we need.