Add LUT-based bitpacking for 1-4 bits

[metascroy]

Summary: Adds new LUT-based bitpacking routine for weight_nbit <= 4.

Reviewed By: digantdesai

Differential Revision: D71357378

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/1987

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 4449732 with merge base 42e1345 ():

NEW FAILURE - The following job has failed:

• PR Label Check / Check PR Labels (gh)
  Process completed with exit code 1.

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

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

• Run TorchAO Experimental Tests / test-mps-ops (macos-m1-stable) (gh) (trunk failure)
  Process completed with exit code 1.

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This pull request was exported from Phabricator. Differential Revision: D71357378