Static attention: do not specialize on input sequence length

[sxu]

Summary: Hardcode embedding/head dims which are fixed and use -1 for the sequence length dimension when reshaping, this allows us to quantize the graph once and export it to different sequence lengths.

Differential Revision: D80181012

[pytorch-bot]

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❌ 1 New Failure, 4 Unrelated Failures

As of commit c972840 with merge base 02a203e ():

NEW FAILURE - The following job has failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (linear, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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This pull request was exported from Phabricator. Differential Revision: D80181012

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This pull request was exported from Phabricator. Differential Revision: D80181012