[StockerMC]

• ☑️ I understand it is strictly prohibited to use AI to write PR description
• AI usage disclosure: I used Claude Code to understand the relevant code paths, help verify the root cause, and write the fix and test. I reviewed, rebuilt and tested everything myself.

fixes #4686

On Metal, the gradient of mx.fast.cross_entropy can be wrong for a non-row-contiguous view like logits[:, :-1] once the input is big enough that the command buffer gets committed right after the VJP. It's then possible for the next buffer to read the gradient before it's written, so calls sometimes fail.

As was mentioned in the issue, ensure_row_contiguous registers the input copy as a temporary, and the VJP then donates that copy to the output. end_encoding drops temporaries from the encoder's outputs, so the gradient buffer isn't tracked by the fence and the next command buffer doesn't wait for it to be written.

This fix no longer registers the input copy as a temporary, since the output shares its buffer, matching the CUDA VJP and LayerNormVJP.

I added a regression test with an input big enough so that the command buffer always splits after the VJP, and I built and tested on an M3 Pro.

[utsav1033]

the script showed that it computed gradient thrice and printed its size, correct being 34.352, which i got right everytime. and that wasnt the case on main as it was 0.0 at first and 6218.577 later.
the new test proved through a test added by this pr, the bug test failed on main and passed on this pr.
nothing else broke when i ran the related test files(test_fast, test_losses, test_autograd, test_vmap)
my setup is apple m4 on macOS 27.0.1