Add copyright header and pre-commit validator

[jainapurva]

To comply with Meta Copyright Policy, added a pre-commit to add/validate it. The PR also contain files updated with the header, and some automatic pre-commit formatting fixes.

Files which contain the updated code for pre-commit:
.pre-commit-config.yaml
scripts/check_copyright_header.py

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/1900

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 2 Pending, 1 Unrelated Failure

As of commit 5e7ae13 with merge base d258a11 ():

NEW FAILURE - The following job has failed:

• Run Float8nocompile Tests / test (SM-89, linux.g6.4xlarge.experimental.nvidia.gpu, --pre torch --index-url https://download.p... / linux-job (gh)
  RuntimeError: Command docker exec -t a2621aa1d24bc590f5b15f69c657bf4dff8cc11ede0b8fff868a0de25c646da6 /exec failed 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)
  ModuleNotFoundError: No module named 'importlib_metadata'

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