[Feature] Implement ensemble reduce

[smorad]

Description

This implements a reduction module for use with ensembles.

Motivation and Context

This relies on #1359 to make progress on https://github.com/pytorch/rl/issues/1344.

• [x] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds core functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)
• [ ] Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [x] I have read the CONTRIBUTION guide (required)
• [x] My change requires a change to the documentation.
• [x] I have updated the tests accordingly (required for a bug fix or a new feature).
• [x] I have updated the documentation accordingly.