.gitignores for ios demo artifacts

[dustinfreeman]

Summary

Following the iOS Demo app instructions generates files that are not tracked in version control. This updates the main .gitignore, and adds a new .gitignore specific to the generated iOS Xcode project.

Test plan

This has been tested, in the sense that I have run through the instructions to the end of the iOS demo without issue, and only needed to make the .gitignore changes so I didn't have the build artifacts appear as untracked in version control.

It is possible these changes may lead to undesired effects in esoteric unofficial workflows with this repo, but I can't know that unless someone speaks up about it. I couldn't find any prior discussion about this.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8153

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[dustinfreeman]

@pytorchbot label "topic: not user facing"

[digantdesai]

Looks good to me. Can you rebase please?
cc @shoumikhin

[dustinfreeman]

Looks good to me. Can you rebase please? cc @shoumikhin

I'm having to manually rebase each time new commits come up on the main branch. You have my permission to rebase it yourself whenever you do the final review.

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[github-actions]

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[shoumikhin]

This demo app now lives at https://github.com/meta-pytorch/executorch-examples/tree/main/mv3/apple/ExecuTorchDemo

[nil-is-all]

This demo app now lives at https://github.com/meta-pytorch/executorch-examples/tree/main/mv3/apple/ExecuTorchDemo

@shoumikhin confirming this PR can be closed now.