Fix _find_device host overhead in tuple/dict recursion - #3283
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Summary: `_find_device` scans kernel arguments for a `torch.device`/`torch.Tensor` to determine the launch device, recursing into nested `tuple`/`list`/`dict` containers. The recursion passed each element directly as `_find_device(item)`. `_find_device` treats its argument as a tuple of args and iterates it (`for arg in args`). Passing a bare `item` that is itself a `torch.Tensor` made the recursive call iterate over the tensor (`for arg in tensor`), materializing the rows of the tensor on the host before the first row matched the `isinstance(arg, torch.Tensor)` check -- a real per-call host overhead on the hot argument-processing path. A 0-dim scalar tensor also fails to iterate at all. Wrap the recursed element in a single-element tuple, `_find_device((item,))`, so the recursion sees exactly one arg and hits the `torch.Tensor` fast path directly. Behavior is unchanged for every previously-working case; the tensor is no longer iterated. Differential Revision: D114780871
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ethche
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Aug 6, 2026
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Summary:
_find_devicescans kernel arguments for atorch.device/torch.Tensorto determine the launch device, recursing into nestedtuple/list/dictcontainers. The recursion passed each element directly as_find_device(item)._find_devicetreats its argument as a tuple of args and iterates it (for arg in args). Passing a bareitemthat is itself atorch.Tensormade the recursive call iterate over the tensor (for arg in tensor), materializing the rows of the tensor on the host before the first row matched theisinstance(arg, torch.Tensor)check -- a real per-call host overhead on the hot argument-processing path. A 0-dim scalar tensor also fails to iterate at all.Wrap the recursed element in a single-element tuple,
_find_device((item,)), so the recursion sees exactly one arg and hits thetorch.Tensorfast path directly. Behavior is unchanged for every previously-working case; the tensor is no longer iterated.Differential Revision: D114780871