Add Pull Request template for functorch

[zou3519]

Stack from ghstack:

• -> Add Pull Request template for functorch #81995
• Turn on linting for functorch #81987
• Delete some no-longer-neccessary debugging output #81990

The functorch in the pytorch/pytorch repository gets synced to the
pytorch/functorch repository.

This PR template tells people to submit PRs against the pytorch in the
pytorch/pytorch repository.

Also beef up our CONTRIBUTING.md

[facebook-github-bot]

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• 🧪  See artifacts and rendered test results at hud.pytorch.org/pr/81995
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[zou3519]

@pytorchbot merge

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here

[github-actions]

Hey @zou3519.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.