Tensor<T> - Fixed missing memory offset from ReadOnlyTensorSpan

[michaelgsharp]

When creating #113166 I didn't have any testing for the implicit conversions and so missed a few places where the memory offset was not being passed through. This fixes that, and makes the memory offset no longer have a default value on the internal constructor so it has to be manually specified every time to not miss it anymore. It also adds testing to catch this case moving forward.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-numerics-tensors
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

This pull request fixes the missing memory offset issue in ReadOnlyTensorSpan by requiring an explicit memory offset in internal constructors and updating all related factory, slicing, and conversion methods. The changes ensure that the memory offset is consistently propagated throughout tensor creation, slicing, and conversion operations, and new tests have been added for verifying the correctness of summing and slicing operations.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Numerics.Tensors/tests/TensorTests.cs	Added tests for Tensor.Sum and updated tensor creation to include memory offset
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor.cs	Updated constructors, conversion operators, and slicing methods to require and propagate memory offset
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor.Factory.cs	Modified factory methods to pass the mandatory memory offset
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorExtensions.cs	Adjusted extension methods to maintain memory offset consistency in tensor broadcasting, reshaping, and other operations

[tarekgh]

Added a small suggestions, LGTM otherwise.

[carlossanlop]

Out of curiosity, how did you catch this? Do you have an open issue that needs to be linked in the PR description, or did you find it by testing manually?

[michaelgsharp]

@carlossanlop I'm working on some more complex slicing operations and during that work I discovered this issue. I decided this was important enough that I'd split it off into its own PR instead of trying to piggy back it on the slicing PR since thats going to be bigger/more complex.

[carlossanlop]

I'm working on some more complex slicing operations and during that work I discovered this issue. I decided this was important enough that I'd split it off into its own PR instead of trying to piggy back it on the slicing PR since thats going to be bigger/more complex.

Cool. Sounds good. I mainly just wanted to learn the process.