Reuse buffers in op handlers

[metascroy]

This PR makes the MLX backend's compound op emitters reuse buffers in place so MLX can donate them, rewriting the batch_norm normalize chain and the sample gumbel/top-p chain to thread results through a small set of temp slots (out==in) instead of allocating a fresh temp per step, while keeping separate slots for multi-use values.

An audit of the remaining emitters/handlers (conv, pooling, gated-delta-rule, SDPA, quantized/gguf linear, and the C++ runtime handlers) confirmed they already reuse buffers or are inherently non-donating (views/fused kernels), so no further changes were needed.

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit e7b94bf with merge base 7e0151e ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  Process completed with exit code 2.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  Process completed with exit code 2.

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