Fix Tensor<T> slice pinning offset

[aw0lid]

Description

Tensor<T>.GetPinnableReference() returned a reference to the first element of the backing array without accounting for the slice's starting offset. As a result, pinning a non-zero-offset slice could expose the parent array's data instead of the slice's data.

Fix

Updated GetPinnableReference() to apply the tensor slice's _start offset using Unsafe.Add:

ret = ref Unsafe.Add(ref MemoryMarshal.GetArrayDataReference(_values), _start);

GetPinnedHandle() already uses GetPinnableReference(), so no changes were required there.

Tests

Added a regression test covering a non-zero-offset tensor slice. The test verifies that:

• The slice contains the expected values.
• GetPinnableReference() points to the first element of the slice.
• GetPinnedHandle() exposes the expected slice data.

closes #134691.

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[aw0lid]

fixed in #134914

[tannergooding]

Sorry for both the delay and not directly looking at this one. There was a number of issues and reports and they needed to be handled end to end, so it was easier to do my own audit and fixes to get everything in "one go".

There's still a couple other minor edges I'm fixing and then separately a perf PR that will be going up.

[aw0lid]

No worries at all! Thanks for the explanation.
Makes sense to handle everything together. Looking forward to the other fixes and the perf PR.