Pass `backend` as positional argument to `SamplerV2` for job mode

[garrison]

This fixes a deprecation warning.

Fixes #632.

[coveralls]

Pull Request Test Coverage Report for Build 9847189901

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 9840717351:	0.0%
Covered Lines:	2410
Relevant Lines:	2410

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💛 - Coveralls