Fix reward of SimpleMultiObsEnv to always be float

[NixGD]

Previously the reward was sometimes returned as an int.

Description

Motivation and Context

Closes #1675

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

Types of changes

• 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)
• Documentation (update in the documentation)

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

Thanks for reporting @NixGD, will be merged as soon as the CI has been passed.

[araffin]

lgtm, thanks =)