Add a Downstream Module

[Pale-Blue-Dot-97]

Is your feature request related to a problem? Please describe.
Downstream experiments of various types need custom task classes to specify the particular setups needed e.g. adapting a model for downstreaming.

Describe the solution you'd like
A module to hold custom task classes for downstream experiments.

Describe alternatives you've considered
Adapt the existing StandardEpoch task with conditional statements and hyperparameters to obtain the customisation required.