Updates the kvcache func in TextDecoder to work with Swift6

[jdowd7]

Refactors this func to address the warning:

Capture of 'keyTensorPointer' with non-sendable type 'UnsafeMutableRawBufferPointer' in a @sendable closure; this is an error in the Swift 6 language mode

• Simplified the index calculations by treating the 4D tensors as 2D (hidden_dim × seq_length)
• Added bounds checking to prevent out-of-range access
• Used direct array indexing with properly calculated linear indices
• Simplified memory copying using direct pointer access for the slices

Tested in our application and its working as expected

[ZachNagengast]

Thanks @jdowd7! Do you have any metrics on if there is a speed difference between the two implementations?

[jdowd7]

Thanks @jdowd7! Do you have any metrics on if there is a speed difference between the two implementations?

Not yet, will benchmark and report back when I get a chance