Treat batch size of 1 as a batch size

[albi3ro]

Context:

Our result type spec does often squeeze out singleton dimensions, such as those arising from a single measurement, or a shot vector of length 1.

But we have an inconsistency around whether or not a batch size of 1, op.batch_size == 1 is considered an actual batch size or a dimension to squeeze out. I would consider a batch size of 1 different than a batch_size of None, and worth maintaining. This was already the behaviour of default.qubit, and is generally how parameter broadcasting works.

Description of the Change:

Eliminates squeezing steps from broadcast_expand.

Benefits:

Consistent, predictable handling of batch dimensions.

Possible Drawbacks:

Someone might have been depending on the squeezing behaviour. That code might now break. This may be risky, especially this close to the release.

Related GitHub Issues:

[sc-68244] Fixes #5977

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