Add torchcodec mock with wav loading and saving

[samanklesaria]

This mock module will allow tests that load and save audio to still work as long as they only load and save wavs (which they all do).

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/audio/3986

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❌ 11 New Failures

As of commit fe375f4 with merge base 15193c1 ():

NEW FAILURES - The following jobs have failed:

• .github/workflows/bandit.yml (gh)
• .github/workflows/integration-test.yml (gh)
• .github/workflows/lint.yml (gh)
• .github/workflows/pr-labels.yml (gh)
• .github/workflows/unittest-macos-cpu.yml (gh)
• .github/workflows/unittest-windows-cpu.yml (gh)
• .github/workflows/unittest-windows-gpu.yml (gh)
• Build documentation / build / Build doc (gh)
• Build M1 Wheels / pytorch/audio / upload / upload-wheel-py3_9-cpu (gh)
  ##[error]Response status code does not indicate success: 429 (Too Many Requests).
• Unit-tests on Linux CPU / tests (3.11) / linux-job (gh)
  ##[error]Response status code does not indicate success: 429 (Too Many Requests).
• Unit-tests on Linux GPU / tests (3.10, 12.6) / linux-job (gh)
  ##[error]Response status code does not indicate success: 429 (Too Many Requests).

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[NicolasHug]

This sounds like a good solution, thank you. It doesn't seem that these mocks are being used in the current state of this PR. Can you show how we'd be using this mock by modifying one or two existing tests?