Metal backend: eliminate memory leak

[manuelcandales]

This pull request refactors the Metal backend's stream commit logic and improves memory management for MPSGraph operations. The most significant changes are the rewrite of the "commit" SyncType to always release the command buffer after commit, and the addition of explicit resource releases to prevent memory leaks after MPSGraph execution.

Stream commit logic changes:

• The ETMetalStream::commit method now always commits and releases the command buffer directly, removing support for the "commit and continue" optimization and adding error logging if no command buffer is present.
• All MPSGraph execution calls in the ops shim now use SyncType::COMMIT instead of SyncType::COMMIT_AND_CONTINUE.

Memory management improvements:

• After MPSGraph execution in all ops (mm_out, convolution, and scaled_dot_product_attention_math_for_mps), the code now explicitly releases the MPSGraph and all associated MPSGraphTensorData objects to prevent memory leaks.

Code simplification and clarity:

• Unused API availability check for scaledDotProductAttentionWithQueryTensor was removed, streamlining the attention math implementation.
• Tensor data variables (biasData, maskData) are now declared outside conditional blocks for proper release and scope management.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Metal backend: Add MPSGraph caching #15346
• -> Metal backend: eliminate memory leak #15343

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15343

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❌ 1 New Failure, 1 Cancelled Job, 2 Pending, 2 Unrelated Failures

As of commit ee043e3 with merge base 5d71c9b ():

NEW FAILURE - The following job has failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t b540883193a2748c3a5dd2c57655318b89e33e008c6e4bb9931f300b86978e97 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• Apple (gh)

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 / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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