Fix TensorExtensions.StdDev

[lilinus]

Fixes #110381

I use IPowerFunctions<T>.Pow(x, 0.5) to replicate sqrt, but special case double and float to use Math.Sqrt and MathF.Sqrt respectively.

Would prefer to add IRootFunctions to method constraints to use T.Sqrt but I don't know if it is possible to add new constraints.

N.b. TensorExtensions is still marked as experimental

[tannergooding]

This should indeed be updated to use IRootFunctions instead; not Pow which isn't guaranteed to behave the same in many important edge cases

[hez2010]

For floating point numbers Pow(0.5) is not the equivalent to Sqrt, so this could result in unexpected behavior for some custom types.
Considering TensorExtensions is still an experimental API, we can take the minor breaking change here to add an additional generic constraint IRootFunctions<T> and use T.Sqrt instead.
@lilinus Would you like to make a follow-up PR for this?