Add short path for LinearOperator.repeat()

[Turakar]

This adds a short path for LinearOperator.repeat() if we would otherwise create a new BatchRepeatLinearOperator which would be a no-op.

repeat() is called like this for example by the constructor of KroneckerProductLinearOperator.

By reducing one level of indirection, we gain two benefits: a) slightly better performance (obviously), and b) some more optimizations by calling native functions instead of default LinearOperator functions, e.g. for _diagonal().

[Balandat]

This makes sense to me. Should we add a unit test for this to check that we indeed return the same object rather than a BatchRepeatLinearOperator in this case?

[gpleiss]

@Turakar please add a unit test like @Balandat suggests, and then we'll merge!

[Turakar]

Seems like there were no unit tests before, so I added tests for both cases: A No-Op case and a true case. I also allowed the repeat(1, 1) case as this is a no-op, too.