Allow post_processing to happen either before or after reverse_one_hot_encoding

[sarthakpati]

Is your feature request related to a problem? Please describe.
Currently, all post processing is done after reverse_one_hot_encoding, but there are a lot of applications that work before this step (e.g., fill holes can work for both, and so can connected components).

Describe the solution you'd like
Allow a mechanism for post processing algorithm developers to specify at which stage they want their method to be called.

Describe alternatives you've considered
N.A.

Additional context
We can expose this logic to the users fairly easily, but since that is not needed ATM, I will leave comments in the code about it.