[Feature Request] Objectives modules refactoring

[Benjamin-eecs]

Motivation

I think we can refactor torchrl.objectives for functional programming.

Solution

maybe change the inheritance of nn.modules, cc @waterhorse1

Checklist

• I have checked that there is no similar issue in the repo (required)

Road Map

[vmoens]

@Benjamin-eecs it was closed automatically because of the linked issues 😅

[Benjamin-eecs]

Yeah, it happens when trying to track multiple related PR, it's alright. May have to manually reopen every time :)