First draft for modular Hindsight Experience Replay Transform

[dtsaras]

Description

I have added the Hindsight Experience Replay Transform specifically implementing the future and last strategy as described in the paper. The transform is a combination of 3 transforms:

• HERSubGoalSampler: It's responsible for sampling indexes for the subgoals and can be changed with another subgoal sampling method for the specific use case.
• HERSubGoalAssigner: It's the method responsible for creating new trajectories given the subgoal indices.
• HERRewardTransform: While it might not necessarily need to be a separate transform yet, it's the method responsible for reassigning the rewards to the newly generated trajectories.

Motivation and Context

It's a modular implementation for hindsight experience replay as requested #1819

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[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2667

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