[decomp] Fix different return type in threshold_backward vs. eager

[cdzhan]

due to type promotion with floating point scalar in decompositions.py

Fixes part of #100838

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit ac323db with merge base 371d8ba ():

NEW FAILURE - The following job has failed:

• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (dynamo_eager_torchbench, 1, 1, linux.g5.4xlarge.nvidia.gpu) (gh)

UNSTABLE - The following jobs failed but were likely due to flakiness present on trunk and has been marked as unstable:

• trunk / linux-focal-rocm5.6-py3.8 / test (default, 1, 3, linux.rocm.gpu, unstable) (gh)
• trunk / linux-focal-rocm5.6-py3.8 / test (default, 3, 3, linux.rocm.gpu, unstable) (gh)

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[ezyang]

@pytorchbot merge -i

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[ezyang]

@pytorchbot merge -i

[pytorchmergebot]

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