docs: warning don’t use torch.profiler.profile context manager around Trainer methods

[KAVYANSHTYAGI]

What does this PR do?

This PR adds a prominent documentation warning to the Trainer class docstring (and optionally Profiler docs) to alert users not to wrap Trainer.fit(), Trainer.validate(), or similar methods inside a manual torch.profiler.profile context manager.
Instead, users are advised to use the profiler argument in the Trainer constructor, which is the robust and officially supported method for profiling with Lightning.

This change aims to prevent a common source of cryptic internal errors and crashes reported by users who attempt to manually profile the Trainer, and improves developer experience by making the correct usage obvious in the docs.

Fixes #20779
(You can fill in the relevant issue if one exists, such as #16958)

This PR does not introduce any breaking changes.

Before submitting

Was this discussed/agreed via a GitHub issue?
Yes, this has been discussed in Lightning issues and user forums (see: #16958 and PyTorch #88472).

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References

PyTorch Issue #88472

Lightning Issue #16958

StackOverflow community discussion

Thanks for considering this! This documentation improvement should help prevent many user headaches and support requests.

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📚 Documentation preview 📚: https://pytorch-lightning--20864.org.readthedocs.build/en/20864/

[KAVYANSHTYAGI]

Thanks for pointing out the confusion. I’ve removed the unrelated Lightning PR and forum links from the references, now only the directly relevant PyTorch issue is cited to support this documentation warning.

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