torch.ormqr: backward support

[nikitaved]

Seems good to have, especially when neither a nor tau requires grads and/or they are pretty small in number.
Fixes #86267

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[lezcano]

Cool code reuse! I left a couple nits.
Could you also add a note in this function saying that the backward is notably slower than that of linalg.qr, so that it doesn't bite people?
After that, feel free to merge

[nikitaved]

@pytorchbot merge

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Hey @nikitaved.
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: ...'.
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