[Bugfix] [Rearrange] Reset the sensors positons correctly version 2

[vincentpierre]

Motivation and Context

This is an alternative fix for #949
In this version, instead of updating the robot in the first observation, we reset the robot manually in each task reset.
This is potentially cleaner since there is no need to change _get_observations but it requires adding a method call in all existing (an future) reset calls of classes that implement the RearrangeTaskv1 class.

How Has This Been Tested

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[aclegg3]

IMO this is the cleaner option. Once sensor redesign is done and robot sensor attachment is refactored, this can hopefully be removed anyway.