Tensor.Slice No Longer Copies

[michaelgsharp]

Previously our Tensor.Slice would copy the underlying data. This was undesirable for many reasons. With these changes we no longer perform a copy of the underlying data for a slice.

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[Copilot]

PR Overview

This PR updates Tensor slicing behavior so that slices no longer copy the underlying data but instead reference the same backing memory.

• Updated test assertions in TensorTests.cs and TensorSpanTests.cs to reflect that slicing now modifies the original tensor data for the sliced region.
• Modified the Tensor implementation in Tensor.cs to introduce a memoryOffset field and adjust the AsTensorSpan and Slice methods accordingly.

Reviewed Changes

File	Description
src/libraries/System.Numerics.Tensors/tests/TensorTests.cs	Updated assertions and comments to match new non-copy slicing behavior
src/libraries/System.Numerics.Tensors/tests/TensorSpanTests.cs	Simplified test assertions by removing redundant span creation
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor.cs	Introduced memoryOffset and updated AsTensorSpan/Slice to avoid data copying

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Using '_values.Length - _memoryOffset' to determine the flattened length of the slice may not correctly reflect the actual number of elements in the sliced tensor. Consider calculating this value from the sliced tensor's dimensions (e.g., using TensorSpanHelpers.CalculateTotalLength(_lengths)) to ensure it accurately represents the slice's data range.

Suggested change

	public TensorSpan<T> AsTensorSpan() => new TensorSpan<T>(ref Unsafe.Add(ref MemoryMarshal.GetArrayDataReference(_values), _memoryOffset), _lengths, _strides, _values.Length - _memoryOffset);
	public TensorSpan<T> AsTensorSpan() => new TensorSpan<T>(ref Unsafe.Add(ref MemoryMarshal.GetArrayDataReference(_values), _memoryOffset), _lengths, _strides, TensorSpanHelpers.CalculateTotalLength(_lengths));