Add ghstack PR detection and error message

[svekars]

Add ghstack detection logic and modified the comment body for clarity.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3740

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure

As of commit 54da101 with merge base 8529ef6 ():

NEW FAILURE - The following job has failed:

• Build tutorials (nightly/test) / build / pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu) (gh)
  RuntimeError: We found a fake tensor in the exported program constant's list. This typically means our tracing system encountered an op that we can't trace through. For the potential source, you can refer to following model attribute: lifted_tensor_1. Please file an issue on github.

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