Add test coverage for futures in ShapleyValueSampling 4/n (#1488)

Summary:
Pull Request resolved: #1488

This diffs adds more testing coverage for attribute_future to ShapleyValueSampling unit tests where forward function returns a scalar per batch, as either a float, integer, 0d tensor or 1d tensor for async methods.

Reviewed By: styusuf

Differential Revision: D68346494

fbshipit-source-id: 7063f3a8e1a52ed78d36786f464a4f8d05f0430d