multigpu: use unet_manual_cast for SelectModelDevice compute dtype

[Kosinkadink]

Follow-up to PR #7063.

What

Replace the hardcoded _force_fp32_cpu_compute helper in SelectModelDeviceNode with _force_supported_compute_dtype, which delegates to comfy.model_management.unet_manual_cast(weight_dtype, device) instead of testing for cpu and casting to torch.float32 by hand.

Why

unet_manual_cast already encodes per-device dtype support:

• CPU: should_use_fp16/should_use_bf16 return False -> cast dtype = torch.float32
• pre-Ampere NVIDIA with bf16 weights: cast dtype = torch.float16
• pre-macOS-14 MPS with bf16 weights: cast dtype = torch.float32
• weight dtype already supported on the device: returns None (no-op)

For the existing SelectModelDevice -> CPU case on an fp16/bf16 model, the behavior is unchanged: unet_manual_cast returns torch.float32 and set_model_compute_dtype casts at use without inflating peak memory. As a bonus, the same code path now handles other 'weight dtype not supported on device' cases without growing the code surface, so the if resolved.type == 'cpu': gate at the call site goes away.

CPU verification trace

weight dtype	unet_manual_cast(weight, cpu)	action
torch.float32	None (early return)	no-op
torch.float16	torch.float32 (fp16/bf16 rejected on CPU)	set fp32 compute
torch.bfloat16	torch.float32 (fp16/bf16 rejected on CPU)	set fp32 compute

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• comfy_extras/nodes_multigpu.py

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📝 Walkthrough

Walkthrough

This PR replaces the CPU-specific helper _force_fp32_cpu_compute with _force_supported_compute_dtype(patcher, device), which queries comfy.model_management.unet_manual_cast for a device-appropriate compute dtype and calls patcher.set_model_compute_dtype only when a cast is needed. SelectModelDeviceNode.execute is updated to call this helper for any resolved non-null device immediately after retargeting the patcher.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: replacing a hardcoded CPU dtype helper with a device-agnostic one that uses unet_manual_cast.
Description check	✅ Passed	The description is directly related to the changeset, explaining the motivation, implementation details, and providing verification traces for the dtype handling behavior.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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