[pyproject.toml] Add a dependency on `torch==2.3`

[dbort]

This is the version that the release/0.2 version of executorch depends on.

Also update the CI setup scripts so it can find this version before it's available on PyPI

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3277

• :page_facing_up: Preview Python docs built from this PR

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:x: 2 New Failures

As of commit 57743df9536de512324b33cc30371e9f2fc9b6ba with merge base d3326a2073dee7baf78044fb3afd0772edbc616a ():

NEW FAILURES - The following jobs have failed:

• Apple / test-demo-ios / macos-job (gh) RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / upload-frameworks-ios (gh) Credentials could not be loaded, please check your action inputs: Could not load credentials from any providers

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