BUG: When there is only one backend available for a merge and error may occur

[sfc-gh-jkew]

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Reproducible Example

Demoed - part of benchmarking.

Issue Description

A ValueError, "Cannot cast to any of the available backends, as the estimated cost is too high. Tried these backends:" will be thrown if there is only one backend passed to the calculator.

Expected Behavior

A default backend should be set. If there are no backends at all, then a ValueError can be thrown.

Error Logs

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Installed Versions

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